Characterization of neutron signal in Si-CsI telescope and measurement of the absolute neutron detection efficiency.

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1 Characterization of neutron signal in Si-CsI telescope and measurement of the absolute neutron detection efficiency. E. Bonnet 1 et M. Parlog 2 1 GANIL 2 LPC For the INDRA-FAZIA collaboration NFS Workshop, 31 Mars-1 Avril 2014, GANIL

2 Motivation RIB facilities should provide in a (near) future very neutron-rich nuclei to perform reaction where a lot of free neutrons are expected to be produced. Next 4π array (FAZIA made of Si-Si-Csi telescopes) dedicated to the detection of charged particles is under construction and good improvement in the isotopic resolution of detected nuclei has been made (up to Ca) To reduce more assumption on missing masses in the reconstruction of reactions, an estimation of the number of neutrons is needed. We want : to measure the neutron detection efficiency of the Cesium Iodide material on the energy range provided by NFS (1-40 MeV) validate the procedure to extract neutron signal in a Si-CsI telescope test the response of a new plastic (EJ ) to neutrons and protons N. Zaitseva et al., Nucl. Instr. Meth. Phys. Res. A 668 (2012) 88

3 Plan How neutron signal is evidenced in INDRA data Response of a typical Si-CsI telescope, identification and neutron extraction GEANT simulation used to validate the neutron extraction Available cross section values of CsI(n,X) reactions Description of the experiment Direct measurement (CsI efficiency) Coincidence measurement (Validation of the neutron extraction)

4 CsI(n,X) reactions and neutron signal Neutron interacts in the CsI via (n,x) reaction If X is a charged particle, signal is read in CsI From analysis of data collected with INDRA array, candidate for neutron signal is the following : Incident neutron No energy deposited in the Silicon (Si) Reactions in the CsI(n,X) give a signal corresponding to the secondary particle (proton, alpha... )

5 Si-CsI telescope : 2 identification methods INDRA data : 181Ta+66Zn@39MeV/A Identification cards drawn from energy losses of charged particles SI_CSI_0705 DeltaE(Si) E(CsI) matrix Pulse Shape Discrimination (PSD) on CsI light signal given a Fast-Slow CsI identification matrix

6 Track neutron signal : How does it work? Identification tools of KaliVeda D. Gruyer, J.D. Frankland Fast-Slow CsI identification matrix : Below rel line : electrons, muons Above red line : proton and heavier charged particles

7 Track neutron signal Step1 : We select only particles located above red line in CsI

8 Track neutron signal Step2 : We use Silicon detector has an anti-veto : we select only particles above red line in CsI AND select no signal part in the Silicon

9 Track neutron signal Step1 Step1 + Step2 At the end, we consider these particles are produced by CsI(n,X) reactions.

10 Track neutron signal After identification procedure, we can estimate the ratio between all particles detected in CsI and the secondary part produced in CsI(n,X) reactions.

11 INDRA experimental multiplicities E613 experiment at GANIL D. Gruyer thesis, GANIL (2014) For this case, ratio of detected neutrons respect to hydrogen and helium particles is around 8% What's needed to go further : Detection efficiency to extrapolate the real neutron multiplicity. Check the contribution of second order effects (dead zone, Si(n,X)... )

12 GEANT3 simulation has been performed to study secondary proton from CsI(n,p) reaction. - Main indication is the key role of the volume of the CsI detector. - 10% of efficiency looking at the secondary proton - Multiple scattering contribute less than 10% to this efficiency Neutron incident CsI Secondary Proton GEANT3 simulation, Thèse J. Benlliure (1995)

13 (n,p) and (n,alpha) cross section on 127 I and 133 CsI. Tabulated Cross Sections from 127I(n,p) 133Cs(n,p) (n,p) less than 20 mb 127I(n,alpha) 133Cs(n,alpha) (n,alpha) less than 10 mb Most of data are for neutron energy around 15 MeV We need to enlarge the energy range

14 First measurement : Direct measurement Neutron beam with Fast Chopper producing pulsed beam with a period of 1µs The CsI detector is put directly in the beam Monitor Flux and Energy spectra NFS pulsed neutron beam 1-40 MeV + Monitoring 5 meters for TOF measurement to get the neutron incident energy CsI Advantage : measurement allowed at all energies Disadvantage : indirect method based on calibrated monitors and angular distribution knowledge

15 First measurement : Coincidence measurement Use neutron beam on CH2 target, and looking at the (n,p) elastic scattering Neutron properties is deduced from kinematics law : After detected the proton in one telescope, knowing its energy and angle, we can deduce the associated neutron energy and angle which should be detected in the complementary telescope. The ratio of detected neutrons to protons will give us directly detection efficiency of CsI. Advantage : clean method, fast chopper not needed Disadvantage : more difficult to access to the highest energies for neutron NFS neutron beam 1-40 MeV CH2 target neutron proton Si Si CsI CsI Vacuum chamber Motor will be needed to move the telescopes at different angles corresponding to different neutron energies

16 Second measurement : Coincidence measurement Use neutron beam on CH2 target, and looking at the (n,p) elastic scattering We intend to replace the CsI crystal with an EJ scintillator in one of the telescopes, in order to systematically characterize the new plastic in terms of efficiency and discrimination (p/n) in the whole range of neutron energies accessible at NFS. Positive results would stimulate one to check, further on, the capability of telescopes in which the silicon is backed by a thick EJ scintillator to identify charged reaction products in both the DE-E matrix and via the PSD procedure in the plastic last stage. neutron-gamma discrimination with sources EJ scintillator N. Zaitseva et al., Nucl. Instr. Meth. Phys. Res. A 668 (2012) 88 Thèse M. Sénoville - LPC

17 First estimation of the expected counting rate shown it's possible Telescope 2x2 cm² at 25 cm Optimisation between : distance, angular resolution and thickness of the CH2 target will be performed...

18 Conclusions We ask to type of measurements with NFS beams: - Direct measurement - Correlation measurement using the (n,p) elastic scattering Absolute neutron detection efficiency of Cesisum Iodide (CsI) cristal and EJ plastic will be measure on the energy range 1-40 MeV In a second step we should be able to extract cross section values for I(n,X), Cs(n,X) reactions. Concerning tests on the EJ plastic, positive results on discrimination between neutrons and protons would stimulate one to check, further on, the capability of telescopes in which the silicon is backed by a thick EJ scintillator to identify charged reaction products in both the DE-E matrix and via the PSD procedure in the plastic last stage. Thank you for your attention!