Performance of THGEM as a Neutron Detector

Introduction

I propose to construct and measure the performance of a fission chamber instrumented with preamplifiers known as a Thick Gas Electron Multiplier (THGEM). This fission chamber is a chamber filled with an inert gas enclosing a fissionable target material, like Uranium or Thorium. A neutron of sufficient energy has the potential to interact with fissionable material producing heavy ions known as fission fragments. The fission fragments within 5 micron of the target's surface may escape the target as ions and ionize the gas in the chamber. Electrons freed from the ionization gas can enter the THGEM preamplifier producing secondary electrons which are directed to collectors using strong electric fields.

A THGEM preamplifier is a perforated fiberglass board (PC board) clad with a conducting material. The design is based upon the Gas Electron Multiplier (GEM) invented by Fabio Sauli in 1997<ref name="Sauli1997">F. Sauli, et al, NIM A386, (1997) 531-534 </ref >. The GEM preamplifier is a 50 micron sheet of kapton that is coated on each side with 5 micron of copper. The copper clad kapton is perforated with 50-100 micron diameter holes separated by 100-200 micron in a staggered array . The THGEM preamplifier is a more macroscopic version of GEM that uses a 2 mm thick fiberglass sheet perforated with holes that are 2 mm in diameter.

Strong electric fields are established by supplying a potential difference between the two sides of the kapton, or the fiberglass for the case of the THGEM. The electric field lines transport liberated electrons through the preamplifier holes. For the GEM foils, the smaller diameter of the hole can provide sufficient amplification using a potential difference of 350 V between the two sides. On the other hand, the THGEM with the larger hole diameter requires a higher potential difference of about 2000 Volts to achieve similar amplifications.

The objective of this work will be to construct a THGEM based ionization chamber. The THGEM will follow a proven design <ref name="Agocs">G. Agocs, B. Clark, P. Martinego, R. Oliveira, V. Peskov,gand P. Picchi,JINST, 3, P020112, 2008 </ref > and use a resistive paste to reduce discharge events. The detector may be made sensitive to neutrons by doping the resistive paste with a fissionable material. The doping step will take place once a working THGEM equipped detector has been demonstrated. This fission chamber-like device will have the advantage of measuring the location of the incident neutrons that induced a fission event within the chamber by measuring the ionization signal using a segmented charge collector.

Chapter One

Gas Quenching

Rewrite the first two sentences so quenching is more clearly described.

Gas quenching is a non-ionizing process occurs when a gas molecules with large cross sections for excitation and vibration states decrease a charged particle energy to create any ionization when the charged particle passes through. Usually, the gas mixture ,contains the ionization event, consists mostly of gas atoms as a main source of electrons and the quenching gas, when the free electrons are scattered after the ionization, their energy is decreased by quenching so the number of secondary electrons becomes less, Consequently, a higher voltage is required to get a gain from this mixture than a medium only has a non-quenching gas.<ref name="Sharma"> A.Sharma,F. Sauli, first Townsend coefficients measurements for argon gas european organization for nuclear research (1993) </ref >

Not only does the quenching process decreases the electron energy, but also decreases the positive ions energy (produced by ionization) when the ions collide with these gas molecules and emits a photon or more from these positive ions. These photons represent the energy loss in a form other than the ionization and also is called Argon escape peak in case of using Argon gas.

Gas quenching experimentally can be measured by evaluating Townsend first coefficients A,B for different gas mixtures. the following table represents the Townsend first coefficients' values for different ratio of Ar/CO2 gas mixtures:<ref name="Sharma"/>

Percentage of CO2	3.7	22.8	87.2	100
A [math] cm^{-1}Torr^{-1} [/math]	5.04	221.1	158.3	145.1
B [math] Vcm^{-1}Torr^{-1} [/math]	90.82	207.6	291.8	318.2
[math] \frac{E}{p} \,\,\, Vcm^{-1}Torr^{-1} [/math]	16.2	21.6	32.9	36.4

The electric field pressure ratio in the last row is the upper limit of the reduced electric field which Townsend's equation fits considering E as a uniform electric field.

Decreasing the discharge in THGEM

Operating THEGM detector carries always a hope that the gain will be maximum, but one of problem that forbids to hit the target is the discharge effect. The discharge effect is the case here you experimentally start observing sparks coming from the detector. Producing these sparks refers to many reasons such as <ref name="bachmann"> Bachmann et al NIM A 479 (2002) 294-308 </ref > breaking down the rigidity of surrounding gas, having avalanche size exceeds Raether limit ( [math] 10^7[/math] electron-ion pairs), separating the electrodes vertically with small distances.

Temperature, humidity, and gas flow externally affect the probability of the transition from the proportional multiplication to discharge at a given potential and on the quality and previous history of the electrodes <ref name="bachmann"/>.

In case of heavily ionizing particles like fission fragments, the increase in gain causes the probability of discharge to increase as shown by the figure, but the probability of discharge can be controlled by the separation distance between the THGEM cards <ref name="bachmann"/>

Fig. Discharge probability as function of gain for double GEM detector

References

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