1. 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

2. 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 , 2005
VIII RPC Workshop

3. 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 , 2005
VIII RPC Workshop

4. 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 , 2005
VIII RPC Workshop

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

6. 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 , 2005
VIII RPC Workshop

7. 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 , 2005
VIII RPC Workshop

8. 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 , 2005
VIII RPC Workshop

9. 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 , 2005
VIII RPC Workshop

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

11. 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 , 2005
VIII RPC Workshop

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

13. 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 , 2005
VIII RPC Workshop

14. 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
Mean
rms
Oct , 2005
VIII RPC Workshop

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

16. 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 , 2005
VIII RPC Workshop

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

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

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

20. 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 , 2005
VIII RPC Workshop