RAPPORT D ACTIVITÉ

(1)

2012 - 2013

RAPPORT D’ACTIVITÉ

2012 – 2013

(2)

Foreword

Nuclear Physics Research

Nuclear structure 2

Nuclear dynamics and thermodynamics 9

Theoretical physics and phenomenology 22

Interdisciplinary Research

Nuclear waste management 29

Medical and industrial applications 36

Group « Interactions Fondamentales et nature du Neutrino » (GRIFON)

Precise correlation measurements in nuclear beta decay 42

High resolution study of low energy charge exchange collisions with a MOT (magneto- optical trapped) target

44

Towards a new measurement of the neutron Electric Dipole Moment (EDM) 26

Search for neutrinoless double beta decay 48

Activités Techniques et Administratives

Service administratif 55

Bureau d’études et mécanique 56

Service électronique et microélectronique 59

Service informatique 66

Service instrumentation 69

Documentation 73

Qualité et soutien aux projets 74

Hygiène et sécurité 75

Diffusion du savoir

Enseignement 77

Formation par la recherche 78

Formation permanente 79

Valorisation 83

Actions de communication 84

Conférences et rencontres scientifiques 86

Informations générales

Personnels permanents 92

Organigramme 93

Chercheurs associés 94

Glossaire 95

Le présent rapport d'activité couvre la période 2012-2013. Malgré une situation financière tendue, il témoigne, nous l'espérons, du dynamisme des équipes de recherche avec le concours remarquable de l'ensemble des services du Laboratoire. Malgré sa taille relativement modeste, le Laboratoire couvre un large ensemble de thématiques qui va de la recherche fondamentale à la recherche interdisciplinaire à vocation sociétale.

Sans être exhaustif, voici un bref résumé de nos activités sur la période considérée :

En physique nucléaire, l'équipe 'Dynamique et Thermodynamique' a poursuivi l'analyse des campagnes de mesures avec le détecteur INDRA portant sur l'étude des réactions nucléaires et prépare les tests sur faisceaux des premiers modules de FAZIA, le futur multidétecteur de particules chargées. Le groupe "Structure" est engagé dans d'ambitieux programmes expérimentaux à RIKEN, au GANIL et à ISAC portant sur les noyaux exotiques riches en neutrons. Il prépare aussi une prochaine expérience à ISOLDE.

En theorie et phénoménologie, le groupe a produit d'importants résultats dans les méthodes Monte Carlo quantiques et dans l'étude de l'équation d'état de la matière nucléaire dans les objets stellaires.

En ce qui concerne le groupe "Interactions Fondamentales et Nature du neutrino", l'expérience nEDM au PSI portant sur la mesure du moment électrique dipolaire du neutron est maintenant dans une phase de prise de données. En même temps, le groupe prépare la phase II de l'expérience. La présente période a vu l'achèvement et la finalisation de la prise de données sur NEMO3 au LSM dédiée à la recherche de l'émission double-beta sans neutrinos. Le groupe est maintenant largement impliqué dans la construction du démonstrateur de SuperNEMO. Les expériences de recherche de courants exotiques dans la décroissance beta menèes au GANIL sont en phase d'analyse. De beaux résultats ont été obtenus en collaboration avec des physiciens atomistes dans le domaine de l'interaction ion-atome et dans l'étude du phénomène de shake-off.

Le groupe "Aval du Cycle" a poursuivi l'expérience GUINEVERRE du programme FREYA sur le réacteur sous-critique VENUS-F au SCK. Il prépare en même temps les expériences sur la future ligne NFS à SPIRAL2. L'équipe "Applications médicales et industrielles" mène ses recherches dans le domaine de la hadronthérapie à travers les programmes France-Hadron et Rec-Hadron. Il participe activement au projet ARCHADE et a aussi initié de très fructueuses collaborations avec le monde industriel.

F OREWORD

(4)

En dehors de l'appui aux projets des équipes de recherche, les services du Laboratoire sont engagés dans des développements propres. En particulier, le système d'acquisition FASTER a atteint maintenant un niveau de maturité qui permet son déploiement sur nos expériences et dans de nombreux projets en France et à l'étranger. Notre contribution au projet SPIRAL2 s'est poursuivie et accrue avec notamment une importante contribution des membres de l'atelier au montage de l'accélérateur. Le développement du RFQ pour SPIRAL2 poursuit son cours. Le savoir-faire acquis dans ce domaine va permettre notre participation au développement de la ligne basse énergie de S3 et dans l'équipement du hall expérimental DESIR.

Le Laboratoire comporte une forte composante d'enseignants-chercheurs. Ces derniers ont la tâche difficile de mener de front leur activité d'enseignement et de recherche. Leurs nombreuses prises de responsabilité dans les formations et diplômes font du Laboratoire un acteur reconnu au sein de l'Université de Caen Basse Normandie et de l'ENSICAEN. Depuis de nombreuses années, nous sommes engagés dans de multiples actions de vulgarisation auprès des jeunes et du grand public.

Cette activité s'est poursuivie à travers diverses manifestations et rencontres. A noter l'accueil d'un nombre de plus en plus grand de stagiaires de tout niveau de formation.

Certaines aspects primordiaux, non 'quantifiables', n'apparaissent pas à la lecture d'un tel rapport.

D'abord l'engagement sans failles des personnels dans les projets du Laboratoire et ce, malgré des perspectives de carrière souvent difficiles. Ensuite, l'excellente ambiance de travail dans une atmosphère conviviale et détendue. Enfin, à l'heure de la multiplication des sources de financement, le haut niveau de cohésion, de mutualisation et d'entraide entre les équipes et les services qui font du Laboratoire bien autre chose qu'un simple 'hôtel à projets'.

C'est un plaisir de remercier l'ensemble des personnes qui ont pris part à l'élaboration de ce rapport.

Une mention particulière à Samuel Salvador pour la mise en œuvre de la partie scientifique et à Sandrine Guesnon pour l'important travail de mise en forme de l'ensemble du document.

En vous souhaitant une bonne lecture, Dominique Durand

Directeur du Laboratoire

(5)

Nuclear structure

Nuclear dynamics and thermodynamics

Theoretical physics and phenomenology

R ESEARCH

N UCLEAR P HYSICS

(6)

Nuclear structure

T

he Nuclear Structure (or “Exotiques”) group is active in the investigation of the structure of neutron-rich nuclei using the probes of direct reactions and decay spectroscopy.

In the direct reaction studies, the structure of light (A<50) neutron-rich nuclei, including haloes, clustering and correlations and shell structure, is explored using energetic radioactive beams. Two different approaches are employed: (i) at high energies (>100 MeV/nucleon) and close to the dripline nucleon “knockout”, breakup, inelastic excitation and Coulomb dissociation; and (ii) at low energies (~5 – 10 MeV/nucleon) and closer to stability nucleon transfer.

The high-energy reaction studies, which have been the main focus of the group’s reaction studies activities over the course of the last two years, are undertaken at the Radioactive Isotope Beam Factory (RIBF) at RIKEN where beam intensities 3 or 4 orders of magnitude higher than elsewhere, are available for the light near dripline nuclei. At the RIBF experiments are carried out, with radioactive beams delivered by the BigRIPS fragment separator, using the ZDS zero-degree spectrometer coupled to the DALI2 NaI array and, since Spring 2012, the SAMURAI spectrometer plus NEBULA neutron array. One of the goals of the group in the next few years is to upgrade, through a doubling of the number of scintillator walls, the NEBULA array (“NEBULA-Plus”) to enable us to exploit to the maximum the unique beams available at the RIBF and to explore, in particular, multi-neutron decaying systems and the most exotic neutron-rich systems accessible. This project is, at the time of writing, the subject of a grant request – “EXPAND” – made to the ANR.

Our complementary transfer reaction studies – typically neutron addition to the beam via (d,p) in inverse kinematics – at lower energies and closer to stability employ at GANIL-SPIRAL1, the TiaRA Si-strip array coupled to the EXOGAM Ge-array and the VAMOS spectrometer. In the near future experiments will be undertaken employing beams, such as¹⁶C, not available with SPIRAL1, prepared using the LISE3 separator.

In the case of our TRIUMF based work, the beams are delivered by the ISAC2 facility (which offers a suite of beams unavailable at SPIRAL1) and the SHARC Si-strip array coupled with the TIGRESS Ge-array is employed for the measurements. Owing to the lack of a suitable spectrometer, zero-degree detection is provided by a thin scintillator plus stopper foil setup developed at LPC. The main priority in the near future at ISAC is the measurement of the d(²⁸Mg,pγ)²⁹Mg reaction which will complement our earlier work on d(^24,26Ne,p)^25,27Ne [1,2] and further help map the transition into the island of inversion around N=20.

The second main theme of the group’s research is centred on the investigation of structure throughβ-decay, and in the context of neutron-rich nuclei, the study ofβ-delayed neutron emission. Presently this activity is focussed on R&D for a new neutron time-of-flight array which has included extensive neutron beam measurements at CEA/DAM-Arpajon. In addition, a proof of principle experiment is under preparation for ISOLDE aiming at a measurement of theβ-delayed two-neutron decay of¹¹Li. Extensive source testing, in particular in terms of investigating neutron-gamma discrimination techniques, digital signal processing and new scintillators, has also been carried out at LPC. This work has benefited greatly from the untiring support of our colleagues at LPC who have developed the FASTER digital acquisition system.

N.L. Achouri, F. Delaunay, S. Leblond, J. Gibelin, F.M. Marqués, N.A. Orr, M. Pârlog, M. Sénoville

Collaboration : D. Durand (LPCC), G. Lehaut (LPCC), M. Colonna (INFN Catania), H. Hamrita (IPN Orsay/CEA Saclay)

*PHD students

(7)

The measurements were accomplished using the SAMURAI spectrometer [6] coupled to the large area neutron array NEBULA [7] and were performed as part of the first phase of SAMURAI experiments following the successful commissioning in Spring 2012. The analysis to date has concentrated on the fragment+neutron channels and, in particular, ¹⁷B+n which is known to exhibit a strongly interacting virtuals-wave threshold state [8]. Beyond the intrinsic physics interest noted above, a well defined threshold state provides an ideal means to validate the calibration and analysis procedures.

In addition to accessing ¹⁸B via proton removal from ¹⁹C, which should populate almost exclusively s-wave strength, the complementary probe of neutron removal from a ¹⁹B beam has been investigated. Fig. 1 shows the reconstructed¹⁷B+n invariant mass (or relative energy) spectra for the two reactions which were undertaken at around 240 MeV/nucleon. As may be clearly seen the proton removal populates a very narrow threshold structure, the form of which is consistent with the strongly interacting s-wave virtual state deduced by Spyrou et al. [8]. The neutron removal, however, in addition to the threshold peak, shows clear evidence for the population of a state or states in the region of 0.5–1 MeV.

The further analysis of these preliminary results is currently underway, including two-proton removal from ²⁰N, which is expected to populate preferentially d-wave strength in ¹⁸B. The analysis of the data sets for the analogue reactions populating

21C – C(²²C,²⁰C+n), C(²²N,²⁰C+n) and C(²³O,²⁰C+n) – are also in progress.

The work outlined here forms part of the PhD thesis of S. Leblond who acknowledges the support provided in terms of a 6 month RIKEN Nishina Center International Program Associate fellowship in 2013.

Structure at and beyond the neutron dripline:

18,19 B and ^21,22 C

Collaboration: Tokyo Institute of Technology (Japan), RIKEN (Japan) and the SAMURAI Collaboration

Fig. 1: Preliminary results for the ¹⁷B+n relative energy spectra obtained for proton and neutron removal reactions at 240 MeV/nucleon.

ANIME: a simulation code for NEBULA

Collaboration: Tokyo Institute of Technology (Japan)

A

s noted in the overview to our group’s activities, our experimental program at RIKEN, which aims to explore the neutron dripline and beyond, relies on the coincident detection of charged fragments using the SAMURAI spectrometer and beam velocity neutrons (E~250 MeV) with the NEBULA multi-element plastic scintillator array. The response of the neutron array is a key element in the analysis of these experiments. There are two existing approaches for the description of this response: 1) GEANT4, that uses intra-nuclear cascade models; 2) MENATE, that describes individually all possible reaction channels on H and C. The former, however, functions very much as a “black box” which is difficult to modify if discrepancies with the data appear, while the latter was developed for energies well below 100 MeV, and for the specific setup of cylinders as for the DEMON array.

T

he investigation of the light neutron-rich dripline nuclei, including in particular those exhibiting haloes, is a central theme of nuclear structure physics. In the present work a series of measurements, aimed at elucidating the structure of the two heaviest candidate two-neutron halo systems,¹⁹B and²²C [3-5], and the associated unbound sub-systems¹⁸B and²¹C, the level schemes of which are critical to the defining the¹⁷B-n and²⁰C-n interactions for three-body models, have been undertaken. In addition to being of direct importance to halo physics,^18,19B and^21,22C are of considerable interest in terms of the evolution of shell-structure far from stability as they span the N=14 and 16 sub-shell closures below doubly-magic^22,24O.

(8)

The reaction kinematics are treated as quasi-free scattering on n/p/αinside C, taking into account the corresponding separation energies and intrinsic momentum distributions. The outgoing neutron angular distribution is calculated from the phenomenological parameterization of the DEMONS code for the scattering off nucleons [9], and as an energy-dependent exponential in cosθfor the other channels. The recoiling protons are tracked as they move through the array with the same geometrical interface as the neutrons, and the energy deposited by all charged particles is transformed into light and propagated to the two photomultipliers coupled to each bar.

We have checked the performance of ANIME with data acquired during the commissioning of SAMURAI+NEBULA using the

7Li(p,n) reaction. As an example, Fig. 2 shows the light output recorded in NEBULA from mono-energetic neutrons at 250 MeV.

The agreement is as good at 200 MeV, as well as for the multiplicity of the number of individual bars hit and the relative angle between them. The latter is essential in order to understand cross-talk in the array – that is, the interaction of one neutron at several points within NEBULA that may mimic the detection of several neutrons. ANIME will be employed in constructing a cross-talk filter for the analysis of reaction channels with more than one neutron in the final state, as well as in our planned extension of NEBULA to 4 walls.

Fig. 2: Light output, Q (MeVee), for 250 MeV neutrons interacting with the NEBULA array. The data are compared with the results obtained

using the ANIME code (dashed line). The contributions of individual reaction channels are shown.

R&D for a new time-of-flight neutron array

Collaboration: CIEMAT-Madrid (Spain), CEA-DIF Bruyères-le-Châtel

O

wing to the large Q_βvalues and the low neutron binding energies of the daughter nuclei, theβ-decay of very neutron- rich nuclei is often followed by the emission of neutrons from unbound states. The detection of such relatively low-energy neutrons (<5 MeV) is therefore crucial to constructing complete decay-schemes. In order to improve the detection performance and, in particular, provide for a multi-neutron detection capability, a new modular time-of-flight array based on discriminating scintillators and coupled to a digital data acquisition system is being developed by our group.

Existing neutron arrays based on large plastic scintillator bars, such as the TONNERRE array [10], present limitations. First, owing to the absence of pulse-shape discrimination, the time-of-flight spectra are contaminated by a background arising from the ambientγ-rays and cosmic muons, which renders the identification and measurement of weak neutron transition delicate.

Furthermore, two-neutron detection is extremely difficult as multiplicity-two events are dominated by random coincidences involvingγand cosmic rays. In addition, TONNERRE suffers from a limited energy resolution, asymmetric lineshapes and a relatively high neutron energy threshold (~300 keV).

The following strategies, as described in earlier reports, have been adopted to overcome these limitations. These include, in particular:

Limited scintillator volumes viewed by large-diameter photomultiplier tubes and digital signal processing to lower the threshold and reduce the lineshape asymmetry.

Relatively thin detectors at increased distances (>2 m) to improve the energy resolution.

To allow for multi-neutron detection by reducing the background with pulse shape discrimination, using liquid scintillators or discriminating solid organic scintillators.

A modular array with variable geometry to limit cross-talk and optimise cross-talk rejection schemes.

In the present work, we have opted for a third approach: the development from scratch of a new code ANIME (“Algorithms for Neutron Identification in Modular Experiments”), that treats the individual reaction channels at higher energy in a relatively simple manner, with a more user-friendly geometrical interface. NEBULA consists of multiple planes of vertical plastic scintillator bars, in which neutrons are tracked until they interact with either H or C. The interaction probability for each reaction channel is calculated using the MENATE_R database, which has been extended beyond 100 MeV.

(9)

Fig. 3: Intrinsic detection efficiency as a function of the neutron energy. The measurements (squares) are compared to simulations (solid and dashed lines) for two different thresholds.

An example of the results obtained for the efficiency measurements are shown in Fig. 3. As may be seen, simulations employing the MENATE code [12] and a revised version of MENATE developed for use within GEANT4, predict efficiencies in good agreement with the data.

In terms of the cross-talk measurements, the setup employed is illustrated schematically in Fig. 4 whereby neutrons scattered from the unshielded module [A] to the shielded one [B] were measured for different relative positions of the two modules. Fig. 5 displays, for 2 MeV incident neutrons, the time-of-flight measured between the two detectors for events identified as neutrons.

Also shown are the results of a MENATE based simulation incorporating only the active volume of each detector. Reasonable agreement was obtained between the experimental and simulated cross-talk probabilities for the full range of incident neutron energies (1–15 MeV) and detector relative positions (θ_AB≈40–90°). The next step that will be undertaken will be to test the efficacy of cross-talk filters on these data, and perform more realistic simulations including the inactive materials such as the detector housing.

Fig. 4: Schematic view of the detector configuration used for the cross-talk measurements.

The module design that has been adopted is based on a BC501 liquid scintillator cell with a diameter of 20 cm and a depth of 5 cm, viewed by a 13 cm photomultiplier tube through a light guide. The design was characterised in a series of measurements undertaken using monoenergetic neutron beams at the CEA-DIF Bruyères-le-Châtel facility and the FASTER digital acquisition developed here at LPC. Intrinsic efficiencies and cross-talk probabilities were measured at several neutron energies in the range of interest (1–15 MeV) in order to validate the simulations and the kinematical cross-talk filters developed for higher energies [11]. These data are the first measurements of cross-talk probabilities below 14 MeV neutrons energy.

(10)

Fig. 5: Time-of-flight between detectors A and B (see Fig. 2) for events identified as a neutron in both detectors, measured with 2 MeV neutrons (blue spectra) for three different angles (q_AB) and an analysis threshold of 100 keVee. The results of a simulation

including only the active volumes are shown by the red spectra.

A proof-of-principle experiment accepted at the ISOLDE facility at CERN is envisaged to be run in the near future. The principal goal of this experiment will be the detection of two β-delayed neutrons in coincidence (from the decay of ¹¹Li which has the highest presently known two-neutron emission probability) and, for the first time, the measurement of their energies and angular correlations. The coincident detection of two neutrons is currently being tested using a ²⁵²Cf source with some 10 neutron detectors, an LaBr₃scintillator for the time-of-flight start and elements of the FASTER digital acquisition system.

In order to investigate the possible utility of a reduced scintillator volume on the neutron-γ discrimination at low energy and to explore scintillators other than the usual liquids, we have undertaken a series of measurements to characterise small cylindrical (5 cm diameter × 5 cm thick) samples of organic scintillators: crystals (p-terphenyl, trans-stilbene), liquid scintillators (BC501A and NE213 for reference purposes, deuterated BC537) and discriminating plastics (EJ-299-33, CP197 from CEA/LCAE). Whereas the crystals show a light yield twice as large as that of the usual liquids, their discrimination performance is not significantly better. In particular, the neutron-γseparation obtained at low energy with liquids and the p-terphenyl crystal are similar. The currently available discriminating plastic scintillators cannot compete with the liquids and the crystals in terms of discrimination. The deuterated BC537 liquid scintillator exhibits a smaller light yield and a poorer quality discrimination as compared to BC501A/NE213, and therefore is not a viable alternative.

Finally, it is worthwhile noting that reducing the diameter of the liquid scintillator cell from 20 cm (see above) to 5 cm provides for a lowering of a factor of two of the threshold at which neutrons can be unambiguously identified.

The work presented here forms part of the PhD thesis of M.

Senoville [13].

Investigation of the compressions modes in unstable Nickel isotopes

Collaboration: GANIL, IPN Orsay, ATOMKI (Hungary), KU Leuven (Belgium), Konan University (Japan), KVI Groningen (Netherlands), MSU (USA), Notre Dame (USA), RNCP (Japan), RIKEN (Japan), USC (Spain)

T

he study of collective excitation modes, such as the Isoscalar Giant Monopole (ISGMR) and Dipole (ISGDR) resonances, has been pursued in stable nuclei over much of the last three decades with the aim of determining the incompressibility (K) of nuclear matter [14]. This fundamental property is of significant importance as it dictates the excitation energies of the compression modes and, in terms of the equation of state, it plays a crucial role in describing nuclear collisions and supernovae resulting from the collapse of very heavy stars. Through extensive experimental and theoretical studies, the incompressibility, K, has been relatively well determined in stable nuclei. The asymmetry term in the expansion of K, however, has been poorly determined, since it requires the investigation of compression modes over a broad isotopic chain. In addition, in exotic nuclei new phenomena are expected to occur, such as pygmy resonances with multipole strengths reflecting the collectivity arising from the neutron or proton-skins relative to the core.

With this goal in mind, and following the first successful measurement of the ISGMR and ISGQR in⁵⁶Ni [15], two experiments have been performed at GANIL using secondary beams produced with the LISE3 separator: a search for the ISGDR in⁵⁶Ni and the measurement of the ISGMR and ISGQR in⁶⁸Ni. The three measurements employed the MAYA active target filled either with deuterium or helium (+quencher) gases following the tests described in Ref. [16].

(11)

The ⁶⁸Ni experiment employed a 50 MeV/nucleon beam of intensity 10⁴pps on both deuterium and helium gas targets.

Excitation energy spectra deduced for theα(⁶⁸Ni,α)⁶⁸Ni* reaction are shown in Fig. 6. The GMR was determined to lie at 21.7±1.9 MeV and evidence for a soft monopole mode, predicted but never observed, was found at 13.2±0.5 MeV. The corresponding angular distributions, analysed using Distorted Wave Born Approximation with Random Phase Approximation transition densities, indicate that the GMR exhausts a large fraction of the energy-weighted sum rule and that neutrons mainly contribute to the soft monopole mode. Both experiments using deuterium and helium gas provided coherent results providing added confidence in or conclusions and demonstrating the relevance of alpha inelastic scattering in inverse kinematics in order to probe both the GMR and soft modes in neutron-rich nuclei. This work formed part of the PhD of M. Vandebrouck [17]

and a manuscript has been submitted for publication in Physical Review Letters.

The analysis of the inelastic scattering of ⁵⁶Ni on helium is currently ongoing in order to locate the ISGDR of ⁵⁶Ni. The excitation energy spectrum for⁵⁶Ni has been reconstructed and simulations have been performed to estimate the angular acceptances and the efficiency of the reconstruction. In order to refine our understanding of the setup, the angular distribution for the elastic scattering of⁵⁶Ni on helium has been derived and a preliminary result is shown in Fig. 7. We note that the minimum in the cross-section is a slightly shifted compared to DWBA predictions and the origins of this discrepancy are now being investigated. This work forms part of the PhD of S. Bagchi (University of Groningen).

Finally, we note that the⁵⁶Ni + He data is also being exploited in order to explore the cluster nature of⁵⁶Ni [18].

Fig. 6: Excitation energy spectrum for the α(⁶⁸Ni,α)⁶⁸Ni* reaction for a) all measured angles and b) for θ_CM=5.5°. In both cases data are fitted with Lorentzian distributions centred at 13.2 (red), 15.7 (blue) and 21.7 MeV (red lines) corresponding to the soft GMR, GQR and GMR, respectively. The

broad peak above 25 MeV (dotted line) corresponds to several additional multi-polarities such as L = 1,3.

Fig. 7: Differential angular distribution for the elastic scattering of ⁵⁶Ni on helium.

The dashed line is the result of a DWBA calculation.

(12)

Publications

The N = 16 spherical shell closure in ²⁴O

Tshoo K., Satou Y., Bhang H., Choi S., Nakamura T. et al.

Physical Review Letters 109 (2012) 022501

Search for Superscreening effect in Superconductor

Ujic P., de Oliveira Santos F., Lewitowicz M., Achouri N.L., Assié M. et al.

Structure of unbound neutron-rich⁹He studied using single- neutron transfer

Al Kalanee T., Gibelin J., Roussel-Chomaz P., Keeley N., Beaumel D. et al.

Physical Review C 88 (2013) 034301

Limited Asymmetry Dependence of Correlations from Single Nucleon Transfer

Flavigny F., Gillibert A., Nalpas L., Obertelli A., Keeley N. et al.

Core excitations and narrow states beyond the proton dripline:

The exotic nucleus ²¹Al

Timofeyuk N.K., Fernández-Domínguez B., Descouvemont P., Catford W.N., Delaunay F. et al.

Well-developed deformation in ⁴²Si

Takeuchi S., Matsushita M., Aoi N., Doornenbal P., Li K. et al.

Comment on "First Observation of Ground State Dineutron Decay: ¹⁶Be"

Marqués F.M., Orr N.A., Achouri N.L., Delaunay F., Gibelin J.

Spectroscopy of ¹⁸Na: Bridging the two-proton radioactivity of

19Mg

Assié M., Santos F. D. O., Davinson T., De Grancey F., Achouri N.L. et al.

Physics Letters B 712 (2012) 198-202

β-delayed neutron emission studies

Gómez-Hornillos M.B., Rissanen J., Taín J.L., Algora A., Kratz K.L.

et al.

Hyperfine Interactions 223 (2012) 185-194

Low-lying neutron f p-shell intruder states in ²⁷Ne

Brown S.M., Catford W.N., Thomas J.S., Fernandez-Dominguez B., Orr N.A. et al.

Resonances in ¹⁹Ne with relevance to the astrophysically important ¹⁸F(p,(\alpha))¹⁵O reaction

Mountford D.J., Murphy A. S. J., Achouri N.L., Angulo C., Brown J.R. et al.

Direct mass measurements of ¹⁹B, ²²C, ²⁹F, ³¹Ne, ³⁴Na and other light exotic nuclei

Gaudefroy L., Mittig W., Orr N.A., Varet S., Chartier M. et al.

Electrostatic mask for active targets

Pancin J., Gibelin J., Goth M., Gangnant P., Libin J.F. et al.

Journal of Instrumentation 7 (2012) P01006

In-beam spectroscopic studies of ⁴⁴S nucleus

Caceres L., Sohler D., Grévy S., Sorlin O., Dombradi Z. et al.

One-proton breakup of ²⁴Si and the ²³Al( p, γ )²⁴Si reaction in type I x-ray bursts

Banu A., Carstoiu F., Achouri N.L., Catford W.N., Chartier M. et al.

Resonances in ¹¹C observed in the ⁴He(⁷Be,α)⁷Be and

4He(⁷Be,p)¹⁰B reactions

Freer M., Achouri N.L., Angulo C., Ashwood N.I., Bardayan D.W.

et al.

One and two neutron removal reactions from the most neutron- rich carbon isotopes

Kobayashi N., Nakamura T., Tostevin J.A., Kondo Y., Aoi N. et al.

Publications

The N = 16 spherical shell closure in ²⁴O

Tshoo K., Satou Y., Bhang H., Choi S., Nakamura T. et al.

Search for Superscreening effect in Superconductor

Ujic P., de Oliveira Santos F., Lewitowicz M., Achouri N.L., Assié M. et al.

Structure of unbound neutron-rich⁹He studied using single- neutron transfer

Al Kalanee T., Gibelin J., Roussel-Chomaz P., Keeley N., Beaumel D. et al.

Limited Asymmetry Dependence of Correlations from Single Nucleon Transfer

Flavigny F., Gillibert A., Nalpas L., Obertelli A., Keeley N. et al.

Core excitations and narrow states beyond the proton dripline:

The exotic nucleus ²¹Al

Timofeyuk N.K., Fernández-Domínguez B., Descouvemont P., Catford W.N., Delaunay F. et al.

Well-developed deformation in ⁴²Si

Takeuchi S., Matsushita M., Aoi N., Doornenbal P., Li K. et al.

Comment on "First Observation of Ground State Dineutron Decay: ¹⁶Be"

Marqués F.M., Orr N.A., Achouri N.L., Delaunay F., Gibelin J.

Spectroscopy of ¹⁸Na: Bridging the two-proton radioactivity of

19Mg

Assié M., Santos F. D. O., Davinson T., De Grancey F., Achouri N.L. et al.

Physics Letters B 712 (2012) 198-202

β-delayed neutron emission studies

Gómez-Hornillos M.B., Rissanen J., Taín J.L., Algora A., Kratz K.L.

et al.

Hyperfine Interactions 223 (2012) 185-194

Low-lying neutron f p-shell intruder states in ²⁷Ne

Brown S.M., Catford W.N., Thomas J.S., Fernandez-Dominguez B., Orr N.A. et al.

Resonances in ¹⁹Ne with relevance to the astrophysically important ¹⁸F(p,(\alpha))¹⁵O reaction

Mountford D.J., Murphy A. S. J., Achouri N.L., Angulo C., Brown J.R. et al.

Direct mass measurements of ¹⁹B, ²²C, ²⁹F, ³¹Ne, ³⁴Na and other light exotic nuclei

Gaudefroy L., Mittig W., Orr N.A., Varet S., Chartier M. et al.

Electrostatic mask for active targets

Pancin J., Gibelin J., Goth M., Gangnant P., Libin J.F. et al.

Journal of Instrumentation 7 (2012) P01006

In-beam spectroscopic studies of ⁴⁴S nucleus

Caceres L., Sohler D., Grévy S., Sorlin O., Dombradi Z. et al.

One-proton breakup of ²⁴Si and the ²³Al( p, γ )²⁴Si reaction in type I x-ray bursts

Banu A., Carstoiu F., Achouri N.L., Catford W.N., Chartier M. et al.

Resonances in ¹¹C observed in the ⁴He(⁷Be,α)⁷Be and

4He(⁷Be,p)¹⁰B reactions

Freer M., Achouri N.L., Angulo C., Ashwood N.I., Bardayan D.W.

et al.

One and two neutron removal reactions from the most neutron- rich carbon isotopes

Kobayashi N., Nakamura T., Tostevin J.A., Kondo Y., Aoi N. et al.

References

[1] W.N. Catfordet al., Phys. Rev. Lett. 104(2010) 192501 [2] S. Brown et al., Phys. Rev. C85(2012) 011302(R) [3] K. Tanaka et al.,Phys. Rev. Lett. 104 (2010) 062701 [4] N. Kobayashiet al.,Phys. Rev. C83 (2012) 054604.

[5] L. Gaudefroyet al.,Phys. Rev. Lett. 109 (2012) 20503.

[6] T. Kobayashiet al.,Nucl. Instr. Meth. B317(2013) 294.

[7] Y. Kondo et al.,RIKEN Accel. Prog. Rep. 45(2012) 131;

http://be.nucl.ap.titech.ac.jp/~nebula

[8] A. Spyrouet al.,Phys. Lett. B 683(2010) 129.

[9] W.C. Sailor etal., Nucl. Inst. Meth. 277 (1989) 599.

[10] A. Buta etal., Nucl. Instr. and Meth. A455 (2000) 412.

[11] F. M. Marqués et al., Nucl. Inst. Meth. A450 (2000) 109.

[12] P. Désesquelleset al., Nucl. Inst. Meth. A307 (1991) 366.

[13] M. Senoville, “Développement d’un nouveau multi-détecteur de neutron”, Thèse, Université de Caen Basse-Normandie (2013) [14] M. N. Harakeh and A. van der Woude, “Giant Resonances:

Fundamental High-Frequency Modes of Nuclear Excitation”, Oxford University Press, Oxford, 2001.

[15] C. Monrozeauet al.,Phys. Rev. Lett. 100(2008) 042501.

[16] J. Pancin et al.JINST 7(2012) 01006.

[17] M. Vandebrouck, “Première mesure des résonances géantes isoscalaires dans un noyau exotique riche en neutrons : le ⁶⁸Ni avec la cible active Maya”, Thèse, Université Paris Sud – Paris XI (2013) http://tel.archives-ouvertes.fr/tel-00872712.

[18] H. Akimuneet al. J. Phys.: Conf. Series436(2013) 012010.

References

[1] W.N. Catfordet al., Phys. Rev. Lett. 104(2010) 192501 [2] S. Brown et al., Phys. Rev. C85(2012) 011302(R) [3] K. Tanaka et al.,Phys. Rev. Lett. 104 (2010) 062701 [4] N. Kobayashiet al.,Phys. Rev. C83 (2012) 054604.

[5] L. Gaudefroyet al.,Phys. Rev. Lett. 109 (2012) 20503.

[6] T. Kobayashiet al.,Nucl. Instr. Meth. B317(2013) 294.

[7] Y. Kondo et al.,RIKEN Accel. Prog. Rep. 45(2012) 131;

http://be.nucl.ap.titech.ac.jp/~nebula

[8] A. Spyrouet al.,Phys. Lett. B 683(2010) 129.

[9] W.C. Sailor etal., Nucl. Inst. Meth. 277 (1989) 599.

[10] A. Buta etal., Nucl. Instr. and Meth. A455 (2000) 412.

[11] F. M. Marqués et al., Nucl. Inst. Meth. A450 (2000) 109.

[12] P. Désesquelleset al., Nucl. Inst. Meth. A307 (1991) 366.

[13] M. Senoville, “Développement d’un nouveau multi-détecteur de neutron”, Thèse, Université de Caen Basse-Normandie (2013) [14] M. N. Harakeh and A. van der Woude, “Giant Resonances:

Fundamental High-Frequency Modes of Nuclear Excitation”, Oxford University Press, Oxford, 2001.

[15] C. Monrozeauet al.,Phys. Rev. Lett. 100(2008) 042501.

[16] J. Pancin et al.JINST 7(2012) 01006.

[17] M. Vandebrouck, “Première mesure des résonances géantes isoscalaires dans un noyau exotique riche en neutrons : le ⁶⁸Ni avec la cible active Maya”, Thèse, Université Paris Sud – Paris XI (2013) http://tel.archives-ouvertes.fr/tel-00872712.

[18] H. Akimuneet al. J. Phys.: Conf. Series436(2013) 012010.

(13)

Nuclear dynamics and thermodynamics (INDRAFAZIA collaborations)

T

he determination of the nuclear equation of state (EOS) is one of the key issue concerning Nuclear Physics. The characterisation of its dependence in term of density, temperature and isospin are mandatory to describe accurately as well heavy-ion collisions and properties of neutron stars. The EOS can be seen as the macroscopic consequence of the properties concerning the underlying nucleon-nucleon (NN) interaction in nuclear matter. Studying the EOS is then directly related to the study of NN interaction, namely its density dependence via many-body correlations, and its isovector properties via the symmetry term of EOS. In order to probe these features, we currently use heavy ion induced reactions in the Fermi energy domain and perform exclusive measurements using the 4π array INDRA. This allows to access to the dynamical (transport properties) and the thermodynamical features of hot and compressed nuclear matter. INDRA is a international collaboration grouping 5 institutes : GANIL Caen, IPN Orsay, LPC Caen, Laval University (Québec) and INFN Napoli (Italy). INDRA is in operation since 1993 and 8 large data takings (campaigns) have been performed at GANIL (stable beams + SPIRAL1/CIME beams) in France and GSI in Germany. The collaboration is composed by 18 physicists + 3 PHD + 1 post-doc (2013) and still continue to maintain INDRA in order to be ready for SPIRAL2 and GANIL beams in a near future. The collaboration is also deeply involved since 10 years on the next-generation 4π array; it is the FAZIA project. Taking advantage from the experience concerning 4π arrays, we are currently developping a new prototype of 4π detector and are in the present time in phase 2 of the FAZIA program. This phase consists in building a fully operational demonstrator, composed of 12 blocks made of 16 identification telescopes Si-Si-CsI with their embedded digital electronics.

Several research topics have been developped in the laboratory concerning the study of the dynamical and thermodynamical properties of nuclei with INDRA as well as instrumental developments for FAZIA. In section 1, we present an analysis concerning the study of transport properties in nuclear matter and the determination of some fundamental in-medium quantities such as the nucleon-nucleon mean free path and cross section. In section 2, we address temperature and excitation energy measurements from an experimental point of view; indeed, these observables are at the centre of any thermodynamical study and therefore also for the accurate determination of the nuclear EOS. In section 3, we present a recent experimental work concerning the evaluation of the symmetry energy term on the nuclear EOS. In section 4, we show an experimental program aiming at the evaluation of the best Pulse Shape Analysis which can be achieved with highly homogeneous silicon detectors for the FAZIA project. At last, Section 5 is devoted to the modelization of current signals produced in Silicon detectors, in order to optimize the Pulse Shape Analysis for FAZIA.

L. Augey

^*1

, R. Bougault, M. Kabtoul

^*2

, E. Legouée

^*3

, N. Le Neindre, O. Lopez, M. Parlog, E. Vient

Collaboration : D. Durand (LPCC), G. Lehaut (LPCC), M. Colonna (INFN Catania), H. Hamrita (IPN Orsay/CEA Saclay)

*PHD students

1since october 2013, ²until july 2013, ³until october 2013

(14)

T

ransport properties are critical in the description of the supernova collapse and the formation of a neutron star [1].

They are also one of the fundamental ingredients for microscopic models [2] and contribute for the determination of the equation of state via the underlying in-medium properties of the nuclear interaction. Transport properties of nuclear matter are probed with heavy-ion induced collisions (HIC) by looking at dissipation phenomena in term of energy and isospin diffusion. In the Fermi energy domain, transport features should exhibit the interplay between Mean-Field (nucleus) and individual (nucleons) effects, especially when looking at the energy dissipation reached in central collisions where the overlap between the two incoming partners becomes maximal [3]. Fig. 1 displays the mean isotropy ratioR_E[3] as a function of the incident energy for6different symmetric systems ; this compilation consists in40experimental determination and illustrates the large body of data available withINDRA. In this study, we have selected central collisions by using the total charge multiplicity as detailed in [3,4]. The data are compared to the expectedR_Evalues for full transparency (blue curve) and full stopping (red curve). In a simple picture for the central collisions consisting in 2 separate Fermi spheres with relative momentum given by:

P_rel=αP_rel⁰, whereP_rel⁰is the relative momentum according to the incident (relative) energy between the 2 incoming nuclei of the reaction. The above-mentioned situations correspond then toα=0for full stopping andα=1for full transparency.

From the isotropy ratio, we evaluate the stopping ratio reached in such central collisions. This latter is computed as the reduced distanced=(R_E-R_E(α=0))/(R_E(α=1)-R_E(a=0))between the 2 extreme scenarii.It is instructive to note that the quantity d^2/3can be nicely scaled as a function of the characteristic size of the systemA^1/3as shown in [4]. This scaling suggests that the stopping ratio, measured byd^2/3,is related to the size of the system; in aGlauberscenario, the stopping is indeed related to the in-mediumNNcross sectionσ_NNand the average distance crossed by the scattered nucleons.Therefore, one can use das an estimate of theNNmean free path λ_NNor the associated cross sectionσ_NNrelated by the simple formula obtained from kinetic theory:λ_NN≈1/ρσ_NN.This is done in Fig. 2 where we plot the estimatedλ_NNfrom the simple formula:λ_NN≈R/d^2/3, withR=r₀A^1/3andr₀=1.25 fm.

In-medium effects in nuclear matter in the Fermi energy range

collaboration with D. Durand and G. Lehaut

Fig. 1: Mean isotropy ratio R_E for protons as a function of incident energy in central collisions. The symbols correspond to different symmetric systems. The blue and red curves are the theoretical predictions for full transparency (blue) and full stopping

(red) respectively. From [4].

(15)

Fig. 2: In-medium mean free path λ_NNas a function of the incident energy. From [4].

We obtain the incident energy dependence concerning the in-medium mean free pathλ_NN. We see thatλ_NNis maximal around E_inc/A=40 MeVwith a typical valueλ_NN=8-9 fm. The decrease observed at lower incident energy is not commented here since we believe that the reduced distancedis not valid when theMean-Fielddissipation is present; in this case, one has to evaluate properly the dissipation reached in central collisions, due to the 1-body dissipation term (friction). At variance, for incident energies larger than40 A·MeV, we consider the sudden approximation used as a reference for full transparency to be valid.

Within this energy range, we observe a clear decrease forλ_NN, from 8-9 fm at40 A·MeVtoward a saturation around4-5 fmat 100 A·MeV. This latter result is in full agreement with both theoretical and experimental values around and above100 A·MeV [5,6,7].

To evaluate the magnitude of the in-medium effects, we then compare theNNcross sectionσ_NNobtained from theλ_NNvalues displayed by Fig. 2. We take into account the important effect due to the quantum nature of the nucleons (Pauli exclusion principle) as recommended in [8]. We obtain the reduction factors shown by Fig. 3.

The reduction factors are found to be quite large, ranging from20%to40%, indicating that in this incident energy range (40- 100 A·MeV), the in-medium effects are far from being negligible and thus should be taken properly into account in any microscopic descriptions such as transport models. The best agreement is found with the phenomenological prescription proposed in [9] by Danielewicz [10]. It is also worthwhile to note that almost all prescriptions seem to converge toward the same value above100 A·MeV.

Fig. 3: In-medium reduction factor for the in-medium NN cross section.

The curves correspond to different parametrizations used in recent theoretical descriptions. From [4].

(16)

Isospin transport during collisions around Fermi energy

A

previous study [11] allowed us to develop an experimental method giving the probability for a particle to be emitted by a hot Quasi-Projectile (QP), during a nuclear reaction around the Fermi energy. We decided to use this information to determine the probability for a particle of not being evaporated. Knowing this last probability, we can then characterize an eventual pre-equilibrium component or a neck emission, when this contribution exists. For two main reasons, the only way to make this work, is to observe symmetric or quasi-symmetric collisions. Firstly, there are principally binary collisions for these systems. Secondly, for obvious reasons of symmetry, the Quasi-Target (QT) should have the same physical behavior than the Quasi-Projectile, consequently the same probability of evaporating a given particle. This study has been done for different nuclear systems studied by the INDRA collaboration in the framework of a PhD [12]. We have wanted also to confirm experimentally the validity of hypotheses done to determine these different contributions and to show the effective quality of this method of isolation. The tool to attain this goal is to study the isospin diffusion and the isospin layout in the velocity space, during a nuclear reaction, for the first time at two dimensions. TheINDRACollaboration has studied during its fifth campaign the quasi-symmetric systemXe+Snat32 A·MeVusing several isotopes of these both nuclei. It has thus used four isotopic combinations:

Neutron Rich System [NN]

Proton Rich System [PP]

Mixed System with Proton rich projectile and Neutron rich target [PN]

Mixed System with Neutron rich projectile and Proton rich target [NP]

To study the way in which the densities of neutrons and protons during a nuclear reaction are distributed in the velocity space (defined in thec.m.), respectively along the parallel and the perpendicular axis to the beam, we have defined Rami's ratios [13] for different elementary squares in this space. These ratios are determined from ratios of ratios of different isotopes as proton/deuteron obtained for the different considered isotopic configurations of collisions [13]. The ratio is normalized to1for a neutron rich zone and to-1for proton rich zone. The ratio is equal to0if there is an equilibration of isospin.

For a specific selection of Quasi-Projectiles (angles defined in the laboratory frame between 4°and 6°and velocity between 0.1cand0.12c, in thec.m.frame), we present, in Fig. 4, for the system¹²⁴Xe+¹²⁴Snat32 A·MeV, the Rami's ratios (obtained with protons/tritons) as a function of the perpendicular and parallel velocities in the c.m. frame. We have kept events with a QPdetected on one side of the beam. The perpendicular component of particles is negative if the particle is at the opposite side of theQP.

Thermometry and calorimetry studies

Fig. 4: Rami's ratios (for p/t) as a function of the perpendicular and parallel velocities in the c.m. frame for the system ¹²⁴Xe +¹²⁴Sn at 32 A•MeV with a specific selection of QP.

(17)

Fig. 5: Bidimensional maps of experimental probabilities for a proton as a function of the perpendicular and parallel velocities in the c.m. frame for the system ¹²⁴Xe +¹²⁴Sn at 32 A⋅MeV with a specific selection of QP.

The mean velocity of the QPis indicated in Fig. 4 as well as the limit between the front and the back in the QP frame. For comparison, we present, for the protons, in Fig. 5 different maps of probabilities in the velocity space for the same different selections of events than for Fig. 4. We observe a remarkable qualitative agreement between the two methods of isolation of the different contributions produced during a deep inelastic reaction around Fermi energy. For example, the blue parts of Fig. 4, corresponding to the QP contribution (important memory of the initial isospin), are completely compatible with the map of probability of being evaporated by theQP. We have the same trend for the other respective contributions. For the studied very peripheral collisions, there is not isospin balance between theQPand theQC. Only the pre-equilibrium component around the c.m velocity, is equilibrated of this point of view as well as the pre-equilibrium, that we observe on the Coulombian circles. It seems also that there is a fast process of fragmentation, keeping an important memory of initial isospin of the nuclei in collision.

Indeed, we find a contribution coming from the target at the front of the emission sphere of theQP(in red in Fig. 4). We have therefore seen that the use of nuclear systems with an important gradient of isospins between the two partners, can be a very interesting tool to use with a 4π setup, to really understand the mechanism and the sequence of a nuclear reaction in the velocity space. By using specifically the Rami's ratio in the velocity space, we have shown the great interest of this representation to study the isospin transport during nuclear reaction and we have moreover confirmed experimentally the validity of our method of probability determination.

(18)

New experimental approaches of the classical caloric curve to study the disintegration of hot nuclei

T

he richness of the set of data, collected by theINDRAcollaboration during the last twenty years, enabled us to build a set of caloric curves for nuclei of various sizes, by using, for the first time, a single experimental set-up and a single experimental protocol. The experimental difficulties met usually to measure the temperature and the excitation energy of hot nuclei created by nuclear reactions, have brought us to approach the calorimetry by a new method and to perform in a different way the usual thermometry of such nuclei. We will therefore present the different caloric curves thus obtained in Fig. 6, for Quasi-Projectiles produced by symmetric or quasi symmetric reactions at different incident energies (systemsXe+Sn,Ni+Ni,Ar+KCl) [12]. For all systems, at all incident energies, a change of behavior is observed, a clear break of slope corresponding to a change of the mode of de-excitation of the hot nuclei.

A certain number of theoretical calculations showed that hot nuclei support an increase of temperature until a maximal temperature, called limiting temperature T_lim, beyond which the nucleus may fragment [14-16]. This disintegration of the hot nuclei is due to Coulomb instabilities. This phenomenon is observed in the framework of our study as we can seen it in Fig. 7 for the systemXe+Sn. Indeed, there is a total agreement between the apparition of a break of slope in the caloric curve and the reach of this limiting temperature.

We thus observe clearly a transition from a nuclear Fermi gas to another state which might be a gas of particles and fragments.

Fig. 6: Experimental caloric curves obtained for different systems and incident energies..

Fig. 7: On the left, experimental caloric curves for the system Xe+Sn at different incident energies.

On the right, evolution of measured temperatures as a function of QP mass for the same systems and energies (the theoretical curves came from the references [15,16]).

QP QP

(19)

Quantifying EOS symmetry energy with Xe+Sn reactions

collaboration with M. Colonna This contribution is part of M. Kabtoul thesis and is related to an accepted article in the European Physics Journal [17].

T

he density (ρ) dependence of the symmetry term of the Nuclear Equation of State can be parameterized as

whereρ₀is the nuclear saturation density. The first term is related to Pauli correlations; the second term is the potential part.

The value of theγ exponent is linked to theasy-stiffness (γ≥1)orasy-softness (γ<1)of the potential part. The value ofγis presently unknown [18].32 A·MeV ^124,136Xe+^112,124Snreactions were studied with the INDRAmultidetector. Only products detected in the forward centre of mass hemisphere are considered. Observables were measured as a function of an impact parameter, considered as a dissipation scale. The scale is given by the total transverse energy of the light charged particles (Z=1and2) detected in the forward c.m hemisphere (∑

12 ). Low (large) transverse energies correspond to peripheral (central) collisions.

Isospin equilibration

I

^sospin,(N-Z)/A, transport tends to equilibrate the isospin content between the projectile and the target. This has been studied as a function of the impact parameter using the isospin transport ratio [19] :

the index Hrefers to the n-rich system (¹³⁶Xe+¹²⁴Sn) andLto the n-poor system (¹²⁴Xe+¹¹²Sn), Mto the mixed reactions

136Xe+¹¹²Snand ¹²⁴Xe+¹²⁴Snwhose totalN/Zare the same. The triton multiplicity has been used as isospin observable (x).

The evolution of R^t with impact parameter is displayed in Fig. 8. We recover the previous section result, i.e. for very peripheral collision no isospin equilibration is observed where as isospin equilibration is reached above a transverseLCP energy of about100 MeVwhich corresponds to impact parameters below6 fm.

Symmetry energy from isospin diffusion

T

he chosen isospin sensitive variable is the fragment (Z>2) multiplicity difference between the ¹³⁶Xe+¹¹²Sn and

124Xe+¹²⁴Snsystems. It is presented as a function of the impact parameter scale in figure 8. Only products detected in the forwardc.m.hemisphere are considered and quasi-fusion events are removed [20]. The measured fragment multiplicity is

Fig. 8: Isospin transport ratio calculated for the measured multiplicity of tritons, for the four Xe+Sn systems at 32A MeV, as a function of dissipation.

(20)

For∑

12 >100 MeV isospin equilibrium is reached as demonstrated above. Thus theQPde-excitation properties are the same for the two systems, in particular the multiplicity of emitted fragments. Therefore the fragment multiplicity difference reduces to the difference between mid-rapidity multiplicities. Assuming that these multiplicities are not modified by the de- excitation stage, the measured fragment multiplicity difference can be directly compared to transport model predictions for primary fragments without any after burner hypothesis. This avoids resorting to a de-excitation code.

Stochastic Mean Field (SMF) [21] calculations were performed using two different parameterizations of the symmetry energy (γ=1and γ≈0.5). If we now compare data and simulated values (Figure 9), it appears that theasy-soft (γ≈0.5) case does not follow the experimental trend, whereas theasy-stiffcalculation well matches the data forb<6fm(∑

12 >100 MeV). For more peripheral collisions, the comparison does not hold because isospin equilibrium is not reached, thus simulations and data diverge.

C

oncerning theFAZIAproject, the group was mainly involved in the capability and improvement of the so-called Pulse Shape Analysis (PSA) for identification of stopped particles in one single silicon detector. During last period, two main achievements were obtained.

Comparison of rear and front side injection for PSA identification

F

or the sameFAZIA telescope (Si 300µm-Si 500µm-CsI) and electronic chain we recorded data, taken in the same beam and target configuration, consisting of particles produced in heavy ion collisions at intermediate energy. The experiment was performed in two steps. The first one with particles entering by the low electric field side (rear side injection) in both silicons while in the second step they encountered first the high electric field (front side injection). This was simply obtained by turning both silicon detectors by180°. The silicon detectors fulfilled the lastFAZIAspecifications obtained during a few years ofR&Din terms of resistivity homogeneity over the whole surface (20x20mm²), adequate crystal cutting along the main axis to avoid channelling and bespoke electronic and digitization chains. In these conditions we were able to fairly compare both configurations to determine the best solution in term of particle identification both in the usual∆E-E(particles punching through the first silicon Si1and stopped in the second Si2) and PSA (for nuclei stopped either in Si1 or Si2) methods.

∆E-E identification technique

I

t has been established that for the standard ∆E-E technique no significant variations of the identification capability between both configurations have been observed. A very good charge separation for all incident particles as well as an equal impressive isotopic discrimination up toZ=23have been obtained with the same good quality criteria [22].

Fig. 9: Difference of the fragment multiplicities for the systems, ¹³⁶Xe+¹¹²Sn and

124Xe+¹²⁴Sn, versus the transverse energy of light charged particles. Close points show the experimental data. Squares are related to SMF calculations using two parametrizations of the symmetry energy. (asysoft with γ=0.5 and asystiff with γ=1)

Pulse Shape Analysis for the FAZIA project

(21)

Pulse shape identification technique

F

or the front side injection configuration, the correlation between the energy and the maximum of the current signal (I_max) does not give any visible identification. All elements merge together in a very compact cloud, corresponding to a strong correlation between the energy and the maximum current. Thus the maximum amplitude of the current signal is not a good PSA variable when the fragments enter through the high electric field side. Regarding the “Energy vs Charge rise-time"

correlations shown in Fig. 10, we obtain in both cases identification maps, although, the shape of the correlation is very different in the two cases. For the front side injection, the charge rise-time continuously decreases with decreasing energy for ions of any Z value. On the contrary, for rear side injection we observe, for a given Z and starting from high kinetic energies, a rise-and-fall trend of the rise-time. For slow ions, this rise-and-fall produces a ridge where all Zvalues merge together, whatever the particle is. In both cases a no-identification zone is visible for each line at low energy, defining aZ- dependent identification threshold. These thresholds will be determined more precisely in the following.

Particle identification thresholds for rear and front configuration

A

t first sight, the rear side injection method may seem more efficient, since it enlarges the ridge range. We need a quantitative way to estimate thePSAidentification thresholds. Therefore we apply the “Figure of Merit" (FoM) protocol for adjacent peaks in the particle identification spectra. TheFoMis defined as:

₁ ₂ ₁ ₂ 2.35

whereµ₁andµ₂are the centroids,σ₁andσ₂the standard deviations of two Gaussians fitted to adjacent peaks. A value of FoM=0.7was conventionally chosen in order to extract a low energy threshold above which we realize a good identification.

In the case of two Gaussians, isolated, of equal intensity peaks, corresponds to a ratio peak/valley=2 and a correct identification of95%of the events (as an example,FoM=1corresponds to99%). The quantitativeFoMmethod was applied to both matrices of Fig. 10 in order to judge the identification quality for both configurations. TheFoM=0.7identification limit criterion was again adopted.

Fig. 10: PSA technique: Energy vs rise-time of the charge signal for particles stopped in the first Silicon (Si1). Particles punching through the detector have been removed. From [22].

Fig. 11: Thresholds expressed in term of range in Silicon material for Z identification with ∆E(300µm)-E technique (black thick line) and

(22)

The identification thresholds are summarized in Fig. 11 in terms of the range in Silicon, where a spectacular improvement on the identification energy threshold for the rear side injection technique is observed (red line and full symbols). For the front side injection case the range for identification varies from a minimum of170µmto about250µm, whereas in the rear side injection case the minimum range presents a continuous increase, from30to150µm.

Under-depleted silicon detectors

Fig. 12: Energy versus Current maximum correlations at different bias voltages..

I

n a recent test, we have explored the identification capabilities of reverse mounted partially depleted detectors. In such a configuration, the fragments enter the detector through an undepleted region, where the electric field is nominally zero. In the following we will focus on PSA via the “Energy vs Current Maximum" method, since for partially depleted detectors it shows the most promising results. In Fig. 12, “Energy vs Current Maximum" correlations are shown for different bias voltages applied to a 500 µm thick Si2 stage. The full depletion voltage is 290 V. An improvement of isotopic separation, above the identification energy thresholds, with decreasing bias voltage can be clearly spotted in the figure.

However the better mass resolution capability comes at the price of higher identification energy thresholds. Visual inspection of Fig. 12 permits to evaluate the identification energy thresholds for different elements, reported as a function of Z in Fig. 13. From Fig. 13, it is apparent that at 105 V and 130 V bias voltages the energy thresholds for charge identification are slightly lower than those for mass identification. We would like to stress that the detector under test did not allow isotopic identification via PSA when biased at full depletion voltage. In fact its doping uniformity is only about6%, while previous tests performed by the Collaboration showed that a doping uniformity of about1% FWHMor less is needed for isotopic identification at depletion bias voltage. On the other hand, when not fully depleted,PSA of detector signals allowed for both charge and mass separation of fragments, though charge identification energy thresholds were higher than at full depletion. Under-biasing the first stage of a ∆E-E telescope, one could still lower the energy thresholds for PSA isotopic identification, palliating thus an eventually poor doping uniformity.

Fig. 13: (Colour on line) Charge identification thresholds estimated from visual inspection of Energy versus Current maximum correlations (empty squares). Thresholds for the ∆E-

E techniques are also shown as filled triangles for 300µm silicon thickness.

(23)

T

he high frequency digitization of the current signals induced by heavy ions in highly resistivity-homogeneous neutron transmutation doped (n-TD) silicon detectors put in evidence the dependence of their shape on the type and energy of the incident particle (Fig. 14) and related it to the characteristic local energy loss|dE/dx|.

This observation was recently interpreted in terms of charge carrier collection in a “dielectric” image. Our simple formalism – developed in collaboration withIPNOrsay – supposes that electrons (e) and holes (h), created along the track of the ionizing particle, are living for a while as exciton-like couples oriented by the electric field reigning in the detector. Multiplied by their volume concentration, the electric moments of these dipoles lead to a supplementary dielectric bulk polarization described by an enhanced relative permittivity ε’_r>ε_r (ε_r=11.7 for silicon) implying a local distortion of the electric field [23]. In a cylindrical geometry, ε’_r is connected to the instantaneous linear density of carriers N(x,t), initially given by N₀(x)=(1/w)

|dE/dx|,(w=3.62 eVbeing the energy pere–hpair creation):

1 kN, ,

¹

The dissociation of the charge carrier couples is supposed to take place with a constant probabilityλper time unit:

!,"

dN

!,"

dt %

²

as long as thee-h pair linear density overpass a threshold, of low valueN_th, the remainder being allowed to break down without any delay. All the separated carriers drift towards the appropriate electrode by inducing, in accord with theShockley- Ramo’stheorem, the current signal characteristic to each particle and eventually allowing its identification. A fit procedure, based on the above equations and three fit parameters:λ,kandN_th, was used to get the best description of the individual shape of the mean signal induced by several ions of known energy – seee.g. Fig. 15.

Description of current signals in Silicon detectors

collaboration with H. Hamrita

Fig 14: Mean current signals induced by ions of about 100 MeV impinging on the rear side of a n-TD silicon detector.

RAPPORT D ACTIVITÉ

2012 - 2013