Sensitivity of Hybrid Resistive Plate Chambers to Low-Energy Neutrons Byungsik Hong Korea Detector Laboratory, Korea University M. Ito, T.I. Kang, B.I. Kim, H.C. Kim, J.H. Kim, K.B. Lee, K.S. Lee, S. Park, M.S. Ryu, K.S. Sim Korea Detector Laboratory, Korea University S.J. Hong Neuroscience Research Institute, Gachon Medical School

Outline Motivation Detector construction and setup Results Conclusions Gd-coated single gap with strip readout Gd-coated double gap with pad readout LiF-coated double gap with pad readout Conclusions Oct. 10-12, 2005 VIII RPC Workshop

Motivation Why do we want to measure low energy neutrons? Crucial to detect anti-personal mines and explosive underground, etc. Possible application to imaging devices If the R&D is successful, then Easy-to-build neutron detector Cheap and large neutron detection system Oct. 10-12, 2005 VIII RPC Workshop

Basic Idea Coating the inner surfaces of electrodes with neutron sensitive materials Conditions for neutron detectors The neutron capture cross section for coating material should be well known in advance over a wide range of energy. The sensitivity should depend mostly on the capture cross section for neutrons. Oct. 10-12, 2005 VIII RPC Workshop

Candidate 1 Interesting isotopes are about 30% in natural Gd2O3. Ref.) M. Abbrescia et al., NIMA 533 (2004) 149. Oct. 10-12, 2005 VIII RPC Workshop

Candidate 2 n + 6Li  α + 3H (Q = 4.78 MeV, Eα = 2.05 MeV, EH = 2.73 MeV ) ~ 950 barns for 25 meV ~ 550 barns for 75 meV Oct. 10-12, 2005 VIII RPC Workshop

Construction of Hybrid RPC Detector Structure (electrodes) Read Out Operation Mode Gd-RPC/ Plain-RPC Single Gap (bakelite) 2D- Strips Streamer Double Gap Pad LiF-RPC (glass) Low Gain Avalanche Ground Hybrid RPC Gap Signal pad (or strip plane) Oct. 10-12, 2005 VIII RPC Workshop

Gd-Coated Single-Gap RPC Gd layer mixing composition: - Gd : PVC : thinner= 8 : 1 : 2.7 - Gd layer thickness: 25㎛ - Bakelite resistivity ~ 2*1010 Ωcm Readout pad x-width: 100mm y-width: 100mm Gd-Coated Double-Gap RPC Gd-Coated Single-Gap RPC Gd layer mixing composition: - Gd : linseed oil : heptane= 2.5 : 1 : 9 - Gd layer thickness: 80㎛ - Bakelite resistivity ~ 2*1010 Ωcm Readout strip x-strip plane thickness: 0.4mm strip width: 2mm -y-strip plane thickness: 1.6mm strip width: 3mm Oct. 10-12, 2005 VIII RPC Workshop

LiF-Coated Double-Gap RPC LiF layer mixing composition: - LiF : linseed oil : heptane 1.7 : 1 : 29 - LiF layer thickness: 20㎛ - Glass resistivity ~ 7*1012 Ωcm Readout pad x-width: 55mm y-width: 55mm Oct. 10-12, 2005 VIII RPC Workshop

Pictures Assembled Single-Gap Gd-RPC Assembled Double-Gap Gd-RPC Assembled Double-Gap LiF-RPC Oct. 10-12, 2005 VIII RPC Workshop

Experimental Conditions 252Cf (T½=2.645 y) source at KAERI ○  Test detectors Installed behind the concrete wall, which shields gammas and also moderates neutrons in an energy region close to the thermal energy Neutron energy ranges between 10 and 560 meV, centering at 75 meV. ○  Neutron rate Measured, independently, by a Bonner sphere at the position of the present data taking R = 44.5 Hz/cm2 on Sept. 8, 2005 Oct. 10-12, 2005 VIII RPC Workshop

Results Single-gap Gd-RPC with random trigger Ar : i-C4H10 = 50 : 50 Oct. 10-12, 2005 VIII RPC Workshop

Forward vs Backward ‘forward’ configuration ‘backward’ configuration Ref.) M. Abbrescia et al., NIMA 533 (2004) 149. Forward vs Backward Neutron intensity decreases with the depth of the Gd layer exponentially. n e- Gd2O3 RPC Bakelite Gd layer Long way to go ‘forward’ configuration ‘backward’ configuration n e- Gd2O3 RPC Backward scattering electrons generate avalanches: always the same thickness. Bakelite Gd layer Essentially no absorption Oct. 10-12, 2005 VIII RPC Workshop

Results - Single-gap Gd-RPC with random trigger + (xy-coincidence) 1 2 3 4 5 6 7 8 Channel Number of neutrons - Single-gap Gd-RPC with random trigger + (xy-coincidence) - Possible imaging application 1 2 3 4 5 6 7 8 Number of strips Number of clusters Entries 310 Mean 2.563 rms 1.163 Oct. 10-12, 2005 VIII RPC Workshop

Results Double-gap Gd-RPC with random trigger n Ar : i-C4H10 = 65 : 35 readout pad gap Oct. 10-12, 2005 VIII RPC Workshop

Results Double-gap LiF-RPC with random trigger Ar : i-C4H10 = 65 : 35 4.0 mV 5.0 mV 6.5 mV 8.0 mV Oct. 10-12, 2005 VIII RPC Workshop

GEANT Simulation Efficiencies per gap (Gd-RPC) Neutron energy (meV) Forward config. GEANT Simulation Efficiencies per gap (Gd-RPC) Neutron energy (meV) Oct. 10-12, 2005 VIII RPC Workshop

GEANT simulation Efficiencies per gap (LiF-RPC) Neutron Energy (meV) Oct. 10-12, 2005 VIII RPC Workshop

Comparison LiF-RPC 4.0 mV 5.0 mV 6.5 mV 8.0 mV ~60% of possible maximum Oct. 10-12, 2005 VIII RPC Workshop

Conclusions We have built and tested the prototype RPC which is sensitive to low energy neutrons (~75 meV). The Gd- and LiF-coated RPCs show stable operation in the operational HV plateau regions. The 2D-strip readout with the spatial resolution of ~2.5 cm is possible. Technical difficulties to be solved Control the surface roughness for large area Rate capability (we may need low resistive electrodes.) Applications in real world Oct. 10-12, 2005 VIII RPC Workshop