3 Detector calibrations
3. Detector calibrations
4.6. Looking for gamma-rays
4.6 Looking for gamma-rays
Energy(MeV)
0 1 2 3 4 5 6 7
Counts
0 20 40 60 80 100 120 140 160
Figure 4.8: 13Be Gamma spectrum detected by Crystal Ball in coincidence with the data showed in the relative energy spectrum (see figure4.7). The spectrum has been fitted to an exponential + Gaussian distribution in or-der to subtract the background. One broad gamma-ray peak can be found around 2-2.4 MeV. The fit set the gamma-ray peak at E=2.17±0.04 MeV.
Unfortunately, the invariant mass method have a drawback, it lacks sensitivity about the energy within the decaying products, just informs about the energy that has been transformed to kinetic energy. To be able to know the13Be structure with full confidence, it is necessary to compare with the possible states in12Be that could be fed in the reaction. The way to do it is checking the prompt gamma ray emitted just after the decay with the Crystal Ball detector (section2.3.2). Figure4.8shows the gamma spec-trum, including a fit to the background using a negative exponential model and a Gaussian distribution for the broad peak around 2-2.4 MeV.
The daughter nucleus 12Be has been studied extensively in recent years, thus we know already what we are looking for. Figure 4.9 shows that mainly there are three possible excited fed states by the13Be break up at 2.1, 2.24 and 2.7 MeV. Although the second one at 2.24 MeV is an isomeric state [82] with a too long life time (τ =357(22) ns [83], τ = 331 (12) ns [84]) for the possibilities of this detector setup. In case of feeding the isomeric state the decay will occur far from any detection chance. It is also worth
men-4. Analysis
Figure 4.9: 12Be energy level structure with dummy feeding from 13Be . The three possible bound excited states of the12Be are shown. The state at 2.24 MeV is 0+, thus decay via γ emission to the ground state is forbidden, further explanation in the text. The states included in13Be just as a hypoth-esis to ilustrate the decaying via neutron emission that feds the12Be states.
This level scheme includes the usually accepted states around 0.4 and 2 MeV( this latter only can decay via neutron emission to the 2.10 MeV state from the upper part of the resonance). The state at 3 MeV is a possibility suggested in several papers.
tioning that this kind of state will not decay to the ground state by gamma emission because both are 0+and such a transition is forbidden. Therefore it only has two ways to decay to the ground state, electron capture or pair production, in this case the nuclei should be full stripped of electrons, thus the first path should not be available, just the second. The other way of decaying from the 2.24 MeV is emitting one low energy gamma to the 2.1 MeV state from where it could decay again to the ground state.
At the gamma spectrum of figure4.8, the fit to the Gaussian-shaped distribution located the broad peak at E=2.17±0.04 MeV with σ=0.49±0.05, whereas the background has been subtracted as can be seen in figure4.10.
The result of the subtraction was fit again in the same region, positioning the gamma peak at 2.15±0.02 MeV with σ=0.15±0.02.
In the only previous 13Be experiment where gamma rays detectors were available [42], they found not only gammas from the first excited state of12Be at 2.1 MeV but also from the state at 2.7 MeV. The available data of this experiment have not shown a peak that can be correlated to that second state.
The next step is to find a correlation between the gamma ray and any of the resonances found in the12Be + n relative energy spectrum (see figure 4.7). The peak has been gated-on and the result (see figure4.11) seems to
4.6. Looking for gamma-rays
Energy(MeV)
0 1 2 3 4 5 6 7
Counts
0 5 10 15 20 25 30
Figure 4.10: The same gamma spectrum of figure4.8after subtracting the background.
Relative Energy(MeV)
0 1 2 3 4 5 6
Counts
0 5 10 15 20 25 30 35 40 45
Figure 4.11: 13Be Relative energy spectrum after gating on the gamma-ray of 2.1 MeV.
4. Analysis
be dominated by the uncorrelated counts below it, as the result is not too different from the ungated spectrum. In order to clean the uncorrelated counts, the gamma spectrum has been gated off-peak on both sides and a mean of the two relative energy spectra has been performed. The result of the latter has been subtracted from figure4.11, and the outcome is in figure 4.12. Several other methods to subtract the uncorrelated13Be events from the relative energy spectrum of the coincidence with the gamma ray at 2.1 MeV have been tried. Although the final result has been similar.
Figure 4.12: 13Be Relative energy spectrum after subtraction of the uncor-related events. The only bins still favoured are the second and the fourth at 0.3 and 0.7 MeV.
Due to the low statistics is not easy to draw a conclusion from figure 4.12. It seems obvious that there is no correlation with any medium or high energy resonance, as the relative energy spectrum seems rather flat above 1 MeV in all the subtraction methods used. The two bins at 0.3 and 0.7 MeV are in general favoured after removing the uncorrelated events, although the bin at 0.5 MeV is suppressed. Hence, it can be understood as both pos-sible resonances located around those bins are partially fed by a13Be state that decay at the 2.1 MeV12Be state. However the second bin of the relative energy spectrum at 0.5 MeV is the highest and in figure4.12is lower than the fourth, as the total amount of counts in the peak area was higher than the amount subtracted from the off-peak events, thus it is possible that this low-energy resonance is still uncorrelated from the gamma.