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Resistive Plate Chambers as thermal neutron detectors
DIAMINE CollaborationWP-2 BARI
M. Abbrescia, G. Iaselli, T. Mongelli, A. Ranieri, R. Trentadue, V. Paticchio
8th Topical Seminar on Innovative Particle and Radiation Detectors
21 - 24 October 2002 Siena, Italy
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Outline of the talk
• Reasons to build RPCs for thermal neutrons and the Gd-choice
• The method used to build Gd-bakelite RPCs
• Expected performance and possible options
• Some experimental results
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Reasons for new thermal neutron detectors
GroundLandmine
252Cf source
Cosmics
and “fast” n RPC
Thermal n
For instance ...The humanitarian demining problem
Neutron Backscattering Tecnique (NBT)
Metal Detectors not effective against anti- personell mines:
The signature of the presence of a mine is an increase in the number of thermal neutrons from the ground
Neutrons are emitted from a 252Cf source and are revealed after interaction in the ground
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Why RPCs for thermal neutron detection?
• Bakelite electrodes• Gap: 2 mm• HV electrodes: graphite 100 m • Operating pressure: ~ 1 Atm• Gas flow: 0.1 vol/ora• Al or Cu read-out electrodes
bakelite resistivity 10 10- 10 12 cm electrodes treated with linseed oilRPCs are easy to build, mechanically robust, light-weighted,
cheap, can cover large surfaces, are adapt for industrial production, etc.
particularly suitable for “on-field” applications
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Neutron DetectionNeutrons can be revealed only
after the interaction in a
suitable material
The choice of the converter is crucial for the performance
of the detector
Production of secondary
ionising particles
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Gaseous detectors with solid converters
Compromise between:
Disadvantage: the particle produced after the conversion has to escape from the converter and enter the detector active volume to be revealed
large conversion probability
large thickness
large escape probability
small thickness
Advantage: high densityHigh macroscopic cross section
= N (N: # cent. of scattering/cm3)
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Natural Gd
Nat. Gd has the following isotopic composition:
As a consequence of the capture process of a thermal neutron, Gd produces, in the 60% of cases, an electron from internal conversion
“interesting” isotopes are about 30%
Mass No. %152 0.2154 2.2155 14.8156 20.5157 15.7158 24.8160 21.8
“complex” energy spectrum
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Reasons for the nat.Gd choice (compared to the “standard candle”)
• Natural Gd is characterized by a thermal neutron (50 kbarn) 12 times larger than 10B (3840 barn) • Produced electron range (15-30 m) is >than ’s (3-4 m)• Beyond E=100 meV, Gd cross section decreases much more rapidly than the one of 10B•For E1 eV it is smaller than the one of 10B.
For application concerning only thermal neutron detection we have preferred Gd to 10B
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The use of Gd as a converter
Gd is a metal, weakly reacting in humid air, where it oxidises. It is cheap, except when required in very thin layers (order of m).
Gadolinium Oxide Gd2O3 (vulg. “Gadolina”) is a white inert powder (easy to handle), with granuli of 1-3 m in diameter, very cheap.
Gadolinium Oxide Gd2O3 (vulg. “Gadolina”) is a white inert powder (easy to handle), with granuli of 1-3 m in diameter, very cheap.
It is difficult and expensive to obtain Gd enriched in 157Gd (material of strategic interest)
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The layer of converter
Mirror surfaces …
It is constituted by Gd2O3 mixed with linseed oil; the mixture is sprayed onto the bakelite electrodes, which are used to build standard RPCs.Linseed oil is standardly used on the inner surfaces of RPCs built with bakelite (but it is deposed in a different way). It is used by the future LHC experiments, by ARGO, OPERA, etc. (also by BABAR…)
Thanks to A. Valentini
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The advantages of the method
4. It can be used for industrial-scale applications (as required for practical uses), and factories have a great experience about it: it is the very same method used to paint cars …
4. It can be used for industrial-scale applications (as required for practical uses), and factories have a great experience about it: it is the very same method used to paint cars …
1. It is possible to obtain extremely uniform layers, with very constant thickness and density
2. The electric properties (surface resistivity) of bakelite electrodes are not altered
3. It is a method easily appliable to surfaces having large dimensions
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The performance with Gd-RPCs
backward e-
Since neutron intensity, in Gd, decreases exponentially, just the “first layer”
“takes part” to the conversion process
Backward e- have always the same thickness to cross
Layer thickness is not important (in the backward configuration)
Bakelite RPC sensitivity to thermal neutrons: about 1/1000
RPC with 10B: ~5% (note that half of are lost into the bakelite)
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The chambers
2 with a different concentration of the oil-Gd2O3 mixture
3 RPCs 10x10 cm2 in dimensions
1 without Gd2O3, used as a reference
GasHigh Voltage
Signal readout
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How the story goes on…The chambers have been brought to Geel, where we could use
GELINAGeel Electron Linear AcceleratorAn e- beam on an Uranium target produces, for Bremsstrahlung, which, in turn, produce, via photonuclear emission, neutrons
Energy: from a few meV to 20 Mev12 flightpaths: from 8 to 200 m
Peak Yield: 4.5x1019 n/s Average Yield 3.4x10x11 n/s
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How the system works
U
e-
RPC
CI
TDC1
TDC2
t0 start DAQ
tn stop to a multihit TDC
CI: two layers of 10B of 0.35 m each
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The chambers at Geel
“Frame” in plastic material (the RPCs are in plastic material too…)
“Backward” configuration
Flightpath=15 m(CI = 13.5 m)
ne-
e-
Gd2O
3
RPC
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Some “raw data”Ionisation Chamber
RPC “HighConc” Gd
Measured:Time Of Flight (t- t0)
Spectra acquired at the same time for RPC and CI
comparison between RPC and CI
Two regions:• thermal n• resonances
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The results (after calibration)
Energy resolution for RPC worse than for CI ...who cares?
• Some peaks are present only in RPC the spectrum:peaks of the Gd cross section
AgW
Spectra in the resonance zone (few eV)• Resonances due to the presence of filters on the beam: Ag, W, Na, S, Co
Ionisation Chamber
RPC “HighConc” Gd
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The thermal neutron region
Relative efficiency:
CI
RPCrel N
N
Conversion efficiency of 10B: well known
“Roughly” 2.5-3
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The background of the measure
How to measure background: use a Cd filter opaque to neutrons with
Ekin< 0.5 eV(Cd “cutoff”)Advantages: •data coming from the same chamber (also for CI)•run in the same conditionsNoise distributed uniformly in time and not in energy
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EfficiencySubtracting the background …
Integral efficiency
Differential efficiency
E
E CI
RPC
N
NE
min
)(
dEE
E CI
RPC
N
NE)(
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Conclusions• We have developed and demonstrated the feasibility of this simple method (useful for practical and industrial applications) but very effective, to make out of RPCs detectors for thermal neutrons• RPC-Gd experimental efficiency is > 10B theoretical maximum eff. >> 10B-RPC experimental efficiency
• Coupling two of these detectors together efficiency reachesabout 3.5-4 eff. CI (analysys in progress)
We are still far from max. th. eff. for Gd-RPC ... Possible improvements: Gd concentration optmisation, linseed oil polimerisation procedure, more layers, ...