Resistive Plate Chambers as thermal neutron detectors

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Resistive Plate Chambers as thermal neutron
detectors
8th Topical Seminar on Innovative Particle and
Radiation Detectors 21 - 24 October 2002 Siena,
Italy

• DIAMINE Collaboration
• WP-2 BARI
• M. Abbrescia, G. Iaselli, T. Mongelli,
• A. Ranieri, R. Trentadue, V. Paticchio

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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
For instance ...The humanitarian demining problem
Metal Detectors not effective against anti-
personell mines
Neutron Backscattering Tecnique (NBT)
Neutrons are emitted from a 252Cf source and are
revealed after interaction in the ground
The signature of the presence of a mine is an
increase in the number of thermal neutrons from
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 oil
RPCs 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 Detection
Neutrons can be revealed only after the
interaction in a suitable material
Production of secondary ionising particles
The choice of the converter is crucial for the
performance of the detector
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Gaseous detectors with solid converters
High macroscopic cross section
Advantage high density
?? ? N (N cent. of scattering/cm3)
Disadvantage the particle produced after the
conversion has to escape from the converter and
enter the detector active volume to be revealed
Compromise between
large conversion probability large thickness
large escape probability small thickness
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Natural Gd
Nat. Gd has the following isotopic composition
interesting isotopes are about 30
As a consequence of the capture process of a
thermal neutron, Gd produces, in the 60 of
cases, an electron from internal conversion
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 gtthan ?s
  (3-4 ?m)
• Beyond E100 meV, Gd cross section decreases
  much more rapidly than the one of 10B
• For E?1 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).
It is difficult and expensive to obtain Gd
enriched in 157Gd (material of strategic interest)
Gadolinium Oxide Gd2O3 (vulg. Gadolina) is a
white inert powder (easy to handle), with granuli
of 1-3 ?m in diameter, very cheap.
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The layer of converter
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)
Mirror surfaces
Thanks to A. Valentini
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The advantages of the method

• It is possible to obtain extremely uniform
  layers, with very constant thickness and density

• The electric properties (surface resistivity) of
  bakelite electrodes are not altered

• It is a method easily appliable to surfaces
  having large dimensions

• 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

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The performance with Gd-RPCs
Bakelite RPC sensitivity to thermal neutrons
about 1/1000
RPC with 10B 5 (note that half of ? are lost
into the bakelite)
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)
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The chambers
3 RPCs 10x10 cm2 in dimensions
1 without Gd2O3, used as a reference
2 with a different concentration of the oil-Gd2O3
mixture
High Voltage
Gas
Signal readout
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How the story goes on
The chambers have been brought to Geel, where we
could use
GELINA Geel Electron Linear Accelerator
An e- beam on an Uranium target produces, for
Bremsstrahlung, ? which, in turn, produce, via
photonuclear emission, neutrons
Energy from a few meV to 20 Mev 12 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
e-
U
RPC
CI
t0 start DAQ
TDC1
TDC2
tn stop to a multihit TDC
CI two layers of 10B of 0.35 ??m each
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The chambers at Geel
Flightpath15 m (CI 13.5 m)
Frame in plastic material (the RPCs are in
plastic material too)
Backward configuration
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Some raw data
Measured Time Of Flight (t- t0)
Ionisation Chamber
Spectra acquired at the same time for RPC and CI
RPC HighConc Gd
comparison between RPC and CI

• Two regions
• thermal n
• resonances

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The results (after calibration)
Spectra in the resonance zone (few eV)
Ionisation Chamber

• Resonances due to the presence of filters on the
  beam Ag, W, Na, S, Co

Ag

• Some peaks are present only in RPC the spectrum
• peaks of the Gd cross section

W
RPC HighConc Gd
Energy resolution for RPC worse than for CI
...who cares?
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The thermal neutron region
Relative efficiency
Roughly ? 2.5-3
Conversion efficiency of 10B well known
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The background of the measure
How to measure background use a Cd filter opaque
to neutrons with Ekinlt 0.5 eV (Cd cutoff)

• Advantages
• data coming from the same chamber (also for CI)
• run in the same conditions

Noise distributed uniformly in time and not in
energy
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Efficiency
Subtracting the background
Integral efficiency
Differential efficiency
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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 gt 10B
  theoretical maximum eff. gtgt 10B-RPC experimental
  efficiency

• Coupling two of these detectors together
  efficiency reaches
• about 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, ...