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CONSTRUCTION OF POLARIZED HEAVY ION SOURCE AT RCNP
M. Tanaka, T. Ohshima, K. Abe, K. Katori, M. Fujiwara, T. Itahashi, H.
Ogata, M. Kondo
To cite this version:
M. Tanaka, T. Ohshima, K. Abe, K. Katori, M. Fujiwara, et al.. CONSTRUCTION OF POLARIZED HEAVY ION SOURCE AT RCNP. Journal de Physique Colloques, 1990, 51 (C6), pp.C6-553-C6-556.
�10.1051/jphyscol:1990671�. �jpa-00230941�
COLLOQUE DE P H Y S I Q U E
Colloque C6, supplbment a u n022, T o m e 51, 1 5 novembre 1 9 9 0
CONSTRUCTION OF POLARIZED HEAVY ION SOURCE AT RCNP
M. TANAKA', T. OHSHIMA**, K. ABE*', K. KATORI*', M. FUJIWARA, T. ITAHASHI, H. OGATA a n d M. K O N D O
Research Center for Nuclear Physics, Osaka University, Mihogaoka 10-1, :baraki. Osaka 567, Japan
Kobe Tokiwa Jr. College, 2-6-2 Ohtani-cho, Nagata, Kobe 653, Japan '"~aboratory of Nuclear Study, Osaka University, Machikaneyama-cho, Toyonaka, osaka 565, Japan
Nous pr6sentons les d6veloppements r6cents dans la construction de la source d'ions lourds polaris6s du RCNP A Osaka. Le principe de la polarisation est un &change de charge et de spin entre un ion lourd trds 6pluch6 et un atome de sodium polaris&. La possibilit6 de polariser des 3 ~ e par cette mkthode a 6t6 d6montr6e exp6rimentalement pour la premidre fois en utilisant la spectroscopie d'une feuille de C.
Abstract - A recent progress in the construction of a polarized heavy ion source at RCNP, Osaka University is presented. A basic principle of the polarization is a spin and charge exchange collision between a highly stripped heavy ion and a polarized sodium atom. The first possible evidence for the 3 ~ e nuclear polarization generated through this method was experimentally demonstrated by means of the beam foil spectroscopy.
I - INTRODUCTION
A possibility of a polarized ion source based on the spin and charge exchange collisions has been and successfully applied as a polarized proton s o u ~ c e ~ ' ~ ' ~ ) . It was also suggested that any heavy ion can also be polarized by this principle. Though no facility ever had heavy ion sources based on the above principle, such heavy ion sources are expected to be one of the most promising polarized ion sources in future. Standing on the above aspect, we started constructing a proto- type of a heavy ion source which can provide any polarized heavy ions. Installing the polarized heavy ion source in a new ring cyclotron facilitys) now being under construction a t RCNP, Osaka University, nuclear physics with polarized heavy ions at an intermediate energy region (E 100 MeV/A) might hopefully be possible.
As the first step we aim at the production of polarized 3 ~ e ions because of easiness as compared to other heavy ions.
Therefore, the discussion hereafter is confined only upon the polarization of 3 ~ e beams.
2 - OUTLINE OF RCNP POLARIZED HEAVY ION SOURCE
In Fig. 1 is shown a top view of the RCNP polarized heavy ion source being now under construction.
3 ~ e 2 + ions extracted from a 2.45-GHz single stage ECR (Electron Cyclotron Resonance) ion source are accelerated up to around 20 keV. They are separated from other ion components by a D1 magnet through electric lenses and focused on a sodium vapor cell, where spin and charge exchange collisions between incident 3 ~ e 2 + ions and polarized sodium atoms occur. The sodium vapor cell is located inside a solenoidal coil which can produce an axial magnetic field up to 3liG in order to make the atomic polarization of sodium atom large7) by decoupling the local fields felt by sodium atoms on the wall surface of the sodium vapor cell. Laser lights for optical pumping and for the polarization monitor of sodium atoms by means of the Faraday rotation method8) are respectively provided by a single mode ring dye laser and a broad band dye laser, both of which are pumped by 5W AI+ ion lasers.
An electron transferred from a polarized sodium atom to an excited state of the 3 ~ e t atom is polarized as a consequence of the spin and charge exchange collision. The excited 3 ~ e + ions then disintegrate t o the ground or metastable states after emitting photons. During the above photon emission process, a certain amount of the electron polarizat,ion is lost when the LS decoupling field is absent. However according to the estimations given by Hinds et al.9), it was suggested that more than 40% of the initial polarization is preserved for hydrogen even in the absence of the decoupling field under certain conditions. 3 ~ e t ions emerging from the sodium cell are then nuclear-polarized through the hyperfine interactions when ions reach the exit of the solenoidal coil. In this case, Polarized 3 ~ e + are then
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990671
COLLOQUE DE PHYSIQUE
Optical Lens Mirror Coil
Analyser Magnet (
Electric Quadrupole
Solenoidal Coil
Electric Quadr~~pole Lens (93)
Photo-mul.
Current Monitor Plate
T o p v i e w of Llze
RCNP
P o l a r i z e d H e a v y I o n S o u r c eFig.1. A top view of the RCNP polarized heavy ion source
separated by a D2 magnet from 3 ~ e 2 + ions passing through the sodium vapor cell without spin and charge exchange collisions and introduced into a section of the polarization detection, where the beam foil spectroscopy is employed10).
Here, the direction of the nuclear polarization is changed from the beam direction to the direction normal to the beam axis adiabatically by the guiding field generated by a combination of an axial magnetic field of a small solenoidal coil located in front of the D2 magnet and a fringing field of the
D2
magnet. In our later measurement, the D2 magnet was replaced with an electrical energy analyzer composing of a couple of spherical condenser electrodes as shown in Fig. 2, thus enabling us the measurement keeping the polarization direction unchanged. In this case a weak magnetic field was applied parallel to the spin direction throughout the region involving the energy analyzer and the detection section of the nuclear polarization in order to avoid the unexpected spin rotation due to the perturbing fields, such as the earth magnet.3 ~ e + ions analyzed by either
Dz
magnet or electric analyzer are focused on a carbon foil with a thickness of 4 ,ug/cm2. A considerable amount of emerging 3 ~ e ions from the foil are neutralized ( 3 ~ e I) but in the excited states.During the neutral 3 ~ e atoms are in flight, the nuclear polarization is periodically transferred to the electron through the hyperfine interactions, which gives rise to the circular polarization of emitting photons. A polarimeter for the measurement of the photon circular polarization is shown in Fig. 3.
3
-
EXPERIMENTAL RESULT AND DISCUSSIONThe present ECR ion source used for the production of 3 ~ e 2 + ions is a type with a single stage and an rf frequency with 2.45GHz mainly from a n economical reason. Therefore it is difficult to expect a fertile production rate of 3 ~ e 2 + ions. Nevertheless, we tried to optimize it by varying, for example, a mirror ratio, gas consumption rate, and rf power.
Further details on the performance of the ECR ion source were presented in ref. 11). It was summarized that a t the best 3.5 epA 3 ~ e 2 + was collected a t FC2 and the ratio relative t o the 3 ~ e + ion current was about 1%.
Regarding opticaI pumping of sodium atoms, the sodium atomic polarizations were rigorously observed7) for various wall materials by changing a laser power, and external magnetic field. In summary, the maximum polarization was 0.4-0.5 with a sodium vapor thickness of 3 X 1013 atoms/cm2, and holding field of 3 kG for the dry-film coated copper
-.
Photomul.Faraday cup
'He+
b;;i,,c
Fig. 2 Electric energy analyzer for polarization measurement Fig. 3 Polarimeter for circular polarization
or pyrex- glass wall.
In order to enable the measurement of the nuclear polarization for spin and charge exchanged 3 ~ e + , photon candidates suitable for the beam foil spectroscopy were searched for by using a monochrometer. I t was found from the photon spectra taken by bombarding a 4 pg/cm2 thick carbon foil with 24 keV 3 ~ e + beams that only the 388.9 nm peaks corresponding to the transitions from the 3 3 ~ J to
z3s1
states in 3 ~ e I were predominated, where J=O, 1 and 2. Since the half lifetime of these transitions (W 105 ns) are longer than the hyperfine coupling periods of the 3 3 ~ ~ states, the polarization transfer from the nucleus to the atomic electrons is expected to occur before the 3 3states ~ ~ disintegrate, which eventually makes the measurement of the nuclear polarization possible through the observation of the circular polarization of the 388.9 nm photons.The ratios of the 388.9 nm photon yield taken with polarized sodium atoms relative to the yield taken with unpolarized sodium atoms were observed. In order to accomplish the reliable measurement of the circular polarizations, the pumping laser light was chopped repeatedly with a 10s repetition time. Tentative values of the ratios are summarized as follows: The adiabatic spin rotation method gives (1.08 f 0.03) and (1.06 f 0.04) for polarized 3 ~ e + beams with incident energies of 24keV and 2OkeV, respectively. On the other hand, through the measurement at the incident 3 ~ ~ e energy of ZOkeV, where the electrical energy analyzer is employed, the ratio is estimated to be (1.02
f
0.01). From the fact that all the ratios deviate from unity exceeding the errors, the 3 ~ e nuclear polarization is obviously produced through the spin and charge exchange collisions. The absolute value of the nuclear polarization can be estimated froin the above results and calculated analyzing powers for the circular polarization of the 388.9 nm photons. The roughly evaluated value of the nuclear polarization is 0.1.4 - CONCLUSION AND FUTURE PROSPECT
It was, for the first time but in a preliminary way, demonstrated through the present work that the 3 ~ e + nuclear polarization was produced as a result of the spin and charge exchange collisions between 3 ~ e 2 + ions and sodium atoms polarized by optical pumping. A magnitude of the nuclear polarization and electric current of 3 ~ e + are roughly order of 0.1 and 50 enA. Since these values indicate that our polarized ion source is still an infant against the practical use, we must increase both the polarization degree and output current.
Regarding the improvement of the polarization degree, more powerful laser systems (if possible plural numbers of dye lasers, whose frequencies are tuned slightly differently each other so as to be covered the whole range of the Doppler-broadened sodium vapor spectra) and the holding field enough strong to decouple the LS coupling, say 3 T are necessary. The use of the SONA'~) field will also increase the polarization by a factor of two. However, one should pay attention to the emittance increase13) which is induced when charge-changing processes occur in the presence of the large magnetic field. Though emittance growth for the polarized proton ion source was eliminated by placement
C6-556 COLLOQUE DE PHYSIQUE
of the proton ion source and the charge- exchange target in the same magnetic field, more refined idea is necessary for the polarized 3 ~ e ion source.
Regarding improvement on increase of the primary 3 ~ e 2 + current, we are going to replace the 2.45GHz ECR ion source now in operation with a high frequency one.
It is also interesting to note that potassium atoms are expected to yield more 3 ~ e + current than sodium atoms because of larger cross-sections for the spin and charge exchange collisions. In this case, a powerful Ti:Sapphire laser recently developed might be effectively applied for optical pumping of potassium atoms.
In the next stage of our project, we must consider how to strip the electron of the polarized 3 ~ e + ion for the cyclotron injection. One way is to use a 3 ~ e + 2S metastable state, because the preferential stripping to the fully stripped state is expected for the metastable state with the low incident energy like our case. In practice, the polarized 3 ~ e 2 t ion source a t the Birmingham University successfully uses this technique14). However it should be considered whether the above technique is actually applicable to our case in the presence of the large magnetic field or not because of the Stark quenching of the 2S metastable stateg) due to the induced electric field (v X B).
Lastly, it should be noted that we could fortunately obtain a part of the financial support from the government for the construction of the injection system of the polarized heavy ion source a t RCNP. Motivated with this support, we started the design study of the injection system which allows the injection of not only the polarized but also unpolarized heavy ions extracted from another high frequency ECR ion source.
References
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