Plusieurs simulations nous ont aidées à déterminer la meilleure configuration expérimentale pour la mesure de la réaction4He(78Kr,γ)82Sr. Une optique point-point-point a été choisie, qui permet une meilleure réjection du faisceau primaire. Deux détecteurs de type galette et un détecteur silicium de- vraient être suffisants pour mesurer et identifier le noyau composé.
Chapitre
3
L’analyse des données
3.1
Abstract
Since it was not possible to use a ∆E detector, our separation of the beam and CN completely relied on the total kinetic energy and on the times of flight (the real time of flight between two MCPs and the time of flight referenced by HF signal from the accelerator).
Unexpected problems with the silicon detectors
The experiment started with serious troubles in the measurement of the energy using the silicon de- tector. While we had very good resolution of 16 keV with three peak alpha source at the same time we were seeing a two energy peaks of the primary beam. It was realized that it could be systematic problem with the silicon detectors since we saw this problem on few other detectors. The explanation for such unexpected behavior is a coupling of two effects : channelling and recombination enforced by the plasma effect. In case of channelling, the stopping power is smaller which means that the range is longer and the density of the deposited electrons and holes is smaller. When there is no channelling, the stopping power is higher, range is shorter and the density of electrons and holes is higher which increases the recombination probability. Moreover, the plasma effect of electrons and holes screens the applied bias on the silicon detector and the probability of the recombination becomes even higher. So two energy peaks correspond to ions subjected and not subjected to the channeling.
Setup of the LISE Wien Filter
The WF set up is very subtle. This is mainly due to the small velocity difference and consequently small velocity acceptance. In order to set the electric and magnetic field of the WF properly, it was decided to use the primary beam slowed down by degrader in order to have the same velocity as the CN. This way we would diminish the possibility for making mistake by calculation of the fields’ strengths. Unfortunately the original degrader was destroyed and the spare one was too thick, so finally we had to recalculate the fields’ strengths. The linearity of the response of the WF was perfect.
Energy and time calibration
We conducted the energy calibration with the primary beam and we obtained the calibration factor 0.01302MeV/bin. We noticed that during the measurement the energy position of the primary beam has been lowering down during the time. The cause could be the high counting rate which was never below 1000Hz. The high counting rate can increase the inverse current which could explain the change of the gain. A high counting rate can also cause local damage to the crystal lattice. Thus, we recalibrates the energy for each group of runs. Regarding the real time of flight we had two referent points, the primary beam and the locus at 84MeV (see figure3.3).
Origin of the locus at 84 MeV
A large number of counts was observed at the energy around 84 MeV. This energy practically cor- responds to78Kr ions with the velocity selected by the WF. The energy loss of the primary beam in the target was ≈ 2MeV and the energy difference between primary beam and the locus was almost 20 MeV whereby the counting rate of the locus 84MeV was higher than 1000 pps indicating relatively high pro- bability of almost 10−6for the generation of these ions. We examined several possibilities of the origin of these ions as : the scattering on the edge of a adhesive tape, the scattering (on the slits) inside the WF and the scattering in the target. These propositions were shown to be not so realistic. The most realistic explanation was the presence of atmospheric dust deposited on the target, since the atmospheric dust particles have diameter of 10 µm and less and the target thickness was only 0.2 µm. On the example of the only one spherical dust particle of PVC of diameter of 2.5 µm attached on the target at the beam spot, we showed that it could produce the given energy loss with given probability.
Search for the yield of the fusion reaction78Kr(α, γ)82Sr
At first glance, there was no any significant grouping on the energy around 94MeV which would correspond to the CN – 82Sr. Therefore, we used more scrutinized search using the image analysis technique “histogram stretching” which is a part of the histogram modelling techniques. In common language we played with the contrast and the smoothing of the image. Nevertheless, this approach didn’t help us to notice any grouping which could be eventually imputed to82Sr. Thus we used the calculated position (volume of interest - VOI) of the CN in the 3D space consisting of the energy, the real time of flight and the time of flight referenced to the high frequency signal. For the correction on the background we took the second volume V2 around the VOI. Since it didn’t give any results we used second order correction by using the third volume V3 around the second volume V2. Nevertheless, this also didn’t give results.
Estimation of the upper limit for the cross section of the alpha capture78Kr(α, γ)82Sr reaction
Although it was not possible to identify 82Sr, we tried to estimate the upper limit of its yield. For the maximal number of hits belonging to82Sr we took the variation of the number of hits in the VOI, which was √7. To estimate the cross section, besides the target thickness and the total irradiation by the beam, it is necessary to estimate the transmission coefficient through the Wien filter. As the WF was not adjusted well and due to very small velocity acceptance (the slits were very closed because of the high counting rate of the ions from the locus at 84 MeV), the transmission coefficient was extremely small ∼ 10−5. Such small transmission coefficient gave unrealistically high upper limit of the cross section of ∼ 10 mb.
3.2
Introduction
L’expérience a commencé avec des troubles graves dans la mesure de l’énergie des ions incidents en utilisant le détecteur silicium. Cela nous a coûté environ 80% du temps de l’expérience (sur un total de 3 jours) pour résoudre ce problème. Ce problème est décrit en détail dans la première section de ce chapitre. Au début de l’expérience nous avons entrepris plusieurs réglages et étalonnages des détecteurs et du filtre de Wien, comme initialement prévu dans le plan du déroulé de l’expérience. Ces parties sont expliquées dans les sections suivantes. Malgré le peu de temps de faisceau disponible, nous avons réussi à