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, ...