Sensitivity of Hybrid Sensitivity of Hybrid Resistive Plate Chambers to Resistive Plate Chambers to Low-Energy NeutronsLow-Energy Neutrons

Byungsik HongKorea 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 UniversityS.J. Hong

Neuroscience Research Institute, Gachon Medical School

Oct. 10-12, 2005 VIII RPC Workshop 2

Outline

1. Motivation

2. Detector construction and setup

3. Results Gd-coated single gap with strip readout Gd-coated double gap with pad readout LiF-coated double gap with pad readout

4. Conclusions

Oct. 10-12, 2005 VIII RPC Workshop 3

Motivation

1. Why do we want to measure low energy neutrons? Crucial to detect anti-personal mines and explosive

underground, etc. Possible application to imaging devices

2. 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 4

Basic Idea

1.Coating the inner surfaces of electrodes with neutron sensitive materials

2.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 5

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

Mass No. %152 0.2154 2.2155 14.8156 20.5157 15.7158 24.8160 21.8

Oct. 10-12, 2005 VIII RPC Workshop 6

Candidate 2n + 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 7

Construction of Hybrid RPC

Ground

Ground

Hybrid RPC Gap

Hybrid RPC Gap

Signal pad(or strip plane)

DetectorStructure

(electrodes)

Read

Out

Operation

Mode

Gd-RPC/

Plain-RPC

Single Gap

(bakelite)2D-

StripsStreamer

Gd-RPC/

Plain-RPC

Double Gap

(bakelite)Pad Streamer

LiF-RPCDouble Gap

(glass)Pad

Low Gain

Avalanche

Oct. 10-12, 2005 VIII RPC Workshop 8

Gd-Coated Single-Gap RPCGd 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

Gd layer mixing composition: - Gd : PVC : thinner= 8 : 1 : 2.7- Gd layer thickness: 25 ㎛- Bakelite resistivity ~ 2*1010 Ωcm

Readout padx-width: 100mm

y-width: 100mm

Gd-Coated Double-Gap RPC

Oct. 10-12, 2005 VIII RPC Workshop 9

LiF-Coated Double-Gap RPC

Readout padx-width: 55mmy-width: 55mm

LiF layer mixing composition:- LiF : linseed oil : heptane 1.7 : 1 : 29- LiF layer thickness: 20 ㎛- Glass resistivity ~ 7*1012 Ωcm

Oct. 10-12, 2005 VIII RPC Workshop 10

PicturesAssembled Single-Gap Gd-RPC

Assembled Double-Gap Gd-RPC

Assembled Double-Gap LiF-RPC

Oct. 10-12, 2005 VIII RPC Workshop 11

Experimental Conditions252Cf (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 12

ResultsSingle-gap Gd-RPC with random trigger

Ar : i-C4H10 = 50 : 50

Oct. 10-12, 2005 VIII RPC Workshop 13

‘forward’ configuration ‘backward’ configuration

ne-

e-

Gd2O

3

RPC

Backward scattering electrons generate avalanches: always the same thickness.

n

Bakelite Gd layer

e-

Essentially no absorption

Forward vs BackwardRef.) M. Abbrescia et al., NIMA 533 (2004) 149.

Neutron intensity decreases with the depth of the Gd layer exponentially.

ne-

e-

Gd2O

3

RPC

n

e-

Bakelite Gd layer

Long way to go

Oct. 10-12, 2005 VIII RPC Workshop 141 2 3 4 5 6 7 80

Number of strips

Num

ber

of c

lust

ers

Entries 310Mean 2.563rms 1.163

Results

1 2 3 4 5 6 7 80

Channel

Num

ber

of n

eutr

ons

- Single-gap Gd-RPC with random trigger + (xy-coincidence)

- Possible imaging application

Oct. 10-12, 2005 VIII RPC Workshop 15

ResultsDouble-gap Gd-RPC with random trigger

Ar : i-C4H10 = 65 : 35

ne-

e-n

readout pad

gap

e-

e-

gap

Oct. 10-12, 2005 VIII RPC Workshop 16

ResultsDouble-gap LiF-RPC with random trigger

Ar : i-C4H10 = 65 : 35

LiF-RPC4.0 mV5.0 mV6.5 mV8.0 mV

Oct. 10-12, 2005 VIII RPC Workshop 17

Eff

icie

ncie

s pe

r ga

p (G

d-R

PC

)

Neutron energy (meV)

GEANT SimulationForward config.

Oct. 10-12, 2005 VIII RPC Workshop 18

GEANT simulation

Neutron Energy (meV)

Eff

icie

ncie

s pe

r ga

p (L

iF-R

PC

)

Oct. 10-12, 2005 VIII RPC Workshop 19

Comparison

LiF-RPC4.0 mV5.0 mV6.5 mV8.0 mV ~60% of

possible maximum

Oct. 10-12, 2005 VIII RPC Workshop 20

Conclusions

1. We have built and tested the prototype RPC which is sensitive to low energy neutrons (~75 meV).

2. The Gd- and LiF-coated RPCs show stable operation in the operational HV plateau regions.

3. The 2D-strip readout with the spatial resolution of ~2.5 cm is possible.

4. Technical difficulties to be solved Control the surface roughness for large area Rate capability (we may need low resistive electrodes.)

5. Applications in real world