Performance of THGEM as a Neutron Detector

Title

The Performance of Thick Gaseous Electron Multiplier (THGEM) Preamplifiers as a Neutron Sensitive Detector.

Introduction

Motivation

Fast neutron detectors have many application in many disciplines in nuclear technology. Fast neutron detectors are used for Homeland security applications, such as neutron imaging for the large size cargo containers, high penetrating neutrons are desirable when efficient fast neutron detectors are available. These detectors are also used for real time measurements of fast neutron beam flux which are used in nuclear reactors such as the Advanced Test Reactor (ATR). The goal of this research is to build and test the performance gaseous electron multipliers preamplifiers, as they are used as fast neutron detectors that has a fissionable material such as U-238 or U-233.

Problem Statement

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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 a 90/10 Ar/[math]CO_2[/math] gas mixture 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: Building the Detector

Using the ESEM for to test the quality of the the procedure for applying ED7100 paste

procedure and results

Chapter Two : Physical Concepts and Processes

Gaseous Medium Physical Concepts

Induced Neutron Fission Fragment

Importance of Fast Neutron Detectors

"Neutron detectors are used in several different applications and are of great importance both in scientific work as well as in industrial applications, such as nuclear reactors where the reactor neutron flux needs to be monitored. The need for neutron detectors have increased because of the decision to build detectors capable of detecting so called "dirty bombs" in all major harbors in the United States. Also, the scientific interest has increased because of the construction of the European Spallation Source outside of Lund, where neutrons will be used to study different kinds of materials." Linus Ros, Lund University: Faculty of Engineering (LTH), April 4, 2011.

Homeland security application and a need for a large detection area.

A need for detectors that has the ability to discriminate gamma radiation.

low cost and economical stable and robust in harsh radiation areas.

References

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