Hybrid Detector(s) for Complete Gamma Ray Spectroscopy S. S. Bhattacharjee, R Raut, S S Ghugre, A K Sinha UGC-DAE Consortium For Scientific Research, Kolkata Centre, R. Palit Tata Institute for Fundamental Research I. Kojouharov, J. Gerl GSI Germany ………………………

Important to have complete information on on the level structure of nuclei in the (N ~ 20) transient region of the nuclear landscape Test of nuclear models Validity of magic numbers ; Appearance, dis-appearance of shell gaps… Level EnergySpinLifetimesParity nn Angular Correlation Polarization Pico -Nano

Nano- second isomers N =19 nuclei

These detectors (Composite HPGe and Scintillators) have so far been used as stand alone detection systems, we propose to use then in conjunction resulting in a powerful composite detection system with several unique advantages γ -ray detection sytems

System Design Goals The goals of the proposed system include: – Energy range of ~ *000 keV – Energy resolution of ~ a few % at 662 keV – Enhanced efficiency – Possibility for polarization measurements – Enhanced timing resolution of ~ few 100 ps – Possibility for position measurements. – Energy – Timing – Spatial

Enhanced energy resolution is feasible using Granular / Segmented Composite HPGe detectors Excellent timing resolution is obtained from fast scintillators. But since the system is a composite system the scintillator should have an enhanced energy resolution – Lanthanum Bromide – LaBr 3 (Ce) Proposed Detector System(s)

LaBr 3 :Ce Co 60 gamma ray pulse height spectra measured with LaBr 3 :Ce at cathode voltages a) HV = -500 V and b) HV = -700 V. P. Dorenbos et al. IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 3, JUNE 2004 In-beam Prompt timing ressolution of ~ 400 ps. P J R Mason et al. Proc. Of Rutherford Cenetenial Conference on Nuclear Physics

We propose to use a Scintillator (LaBr3) as the Compton scatterer backed by a HPGe, into which we expect to have the maximum energy deposition. The Compton scattering in the scintillator should be such that it should provide enough signal for a reasonable timing measurement, while the energy resolution would be essentially dominated by the energy deposition in the HPGe

Optimization of detector specifications We would have to optimize the following operational parameters – The Compton interaction should deposit low energy in the Scattering Detector. – The maximum energy should be deposited in the Analyser Detector. – Hence, we need to fix /choose the detector thickness as well as inter detector distance.

We have simulated the expected performance of this novel composite detector, using the GEANT4 toolkit and the analysis of the spectra were performed using the AIDA tools. Prior to the actual simulations, the expected performance of the individual detector components such as the LaBr3 and the HPGe were simulated and the response was in agreement with the reported values. We have simulated successfully the performance of a standard 3” x 3” LaBr 3 crystal whose absolute full-energy efficiency at 1332 keV was obtained which is in excellent agreement with the reported value. A similar exercise was carried out to validate characteristics of the HPGe detector material. The simulated mass attenuation co-efficient for the Compton process over the energy range of interest indicated a nearly constant energy response, which would be exploited in the proposed Composite detector. The simulations were performed wherein the HPGe detector used was identical to one of the single crystals of the clover detector, and in the other set a planar HPGe was simulated.

The reported energy resolutions were incorporated into the simulations. // Constants for LaBr 3 energy resolution from 1.5" x 1.5" LaBr3 G4double A1 = ; G4double B1 = ; G4double C1 = E-05 ; G4double D1 = E-09 ; G4double TX1 = ((D1dep)/keV); G4double per_res1 = ((A1 + (B1*TX1) + (C1*TX1*TX1)+(D1*TX1*TX1*TX1))/2.363); G4double sigma1 = per_res1; // Resolution of HPGe Loat et al. Journal of Science & Mathematics G4double A2 = ; G4double B2 = ; G4double C2 = e-7 ; G4double TX2 = ((D2dep)/keV); G4double per_res2 = (sqrt(A2 + (B2*TX2) + (C2*TX2*TX2)))/2.363; G4double sigma2 = per_res2;

In this configuration we have used a – 68mm  68mm  10mm LaBr3 scintillator – HPGe cube of 68 mm  68 mm  20 mm. The gap between the two detectors was about mm. The isotropic source was placed at a distance of 25 cm from the front detector. Configuration -IConfiguration -II In this configuration we have used 43 mm  10mm LaBr3 scintillator ; single Clover crystal (50 mm  70mm) The gap between the two detectors was about mm. The simulations using a thin Si detector for detecting back scattered photons in front of the LaBr3 are in progress.

E1 E HPGe E2 E Scintillator E HPGe V/s E scintiilator for E  = 662 keV

The top spectrum corresponds to the condition with LaBr 3 ≥ 50 keV and the lower spectrum corresponds to the unconditional sum spectra, for a 662 keV gamma-ray

Scintillator

Tapered crystal Regular shaped crystal Light Guide

14.5 mm x 13.7 mm x 10 mm BrilLanCe 380 crystal Coupled to APD, Hamamatsu S , Through a light guide 14.5 mm x 13.7 mm at the base, tapering to 10 mm x 10 mm at the surface

4 x 4 array of BrilLanCe 380 crystals 4 x 4 array of BrilLanCe 380 crystals (14.5 mm x 13.7 mm x 10 mm) with each crystal optically isolated from one another. Each crystal is coupled to a light-guide which is coupled to an APD (Hamamatsu S ) along with the read-out electronics.

The proposed system will allow us the following unique advantages – Comparable energy resolution – Excellent timing resolution – Excellent spatial resolution – Thus could provide us a filter for locating the source of the gamma rays and will help us weed out the background!!!

Questions or comments?