Performance of THGEM as a Neutron Detector
Title
The Performance of a Fission Chamber equipped with Gaseous Electron Multiplier (GEM) Preamplifiers.
Abstract
I propose to construct and measure the performance of a fission chamber instrumented with preamplifiers known as a Gas Electron Multiplier (GEM). This fission chamber is a chamber filled with a 90/10 Ar/ 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 GEM preamplifier producing secondary electrons which are directed to collectors using strong electric fields.
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 . Then, THGEM preamplifier is designed as a macroscopic version of GEM that uses a perforated fiberglass board (PC board) clad with a conducting material. A thick fiberglass sheet, that may have up to 10mm thickness, is perforated with holes with a diameter of 2 mm.
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 is to construct a GEM based ionization chamber. The GEM will follow a proven design (change the reference)<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 will be made sensitive to neutrons by doping the resistive paste with a fissionable material. The doping step will take place once a working GEM 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.
Introduction
Motivation
Fast neutron detectors have many applications in different disciplines of 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. They 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 economically build and test the performance gaseous electron multipliers preamplifiers, as they are installed in detector's chamber that has a coated layer of fissionable material such as U-233.
Problem Statement
Decrease time needed to detect fission fragment. (increase the cathode voltage)
Determine the pulse length of a signal from the GEM detector and compare to a fission chamber.
Rad hard pre amplification.
Detecting neutrons with E > 1 MeV?
Theory
The detector operation has successive physical processes that governs its performance. The beginning is a neutron induced fission that occurs on U-233 coating on the cathode, the fission produces two fission fragment moving back to back,at least one of them will escape from the surface of U-233 coating into the surrounding gas. The fission fragment will move, collide, and ionize the gas depending on its energy and the gas cross section of ionization, it will exhange charge with the medium's atoms and molecules until it becomes neutral, meanwhile electrons will primarily scatter but most of them will diffuse in the direction of drift electric field which guide them to the first GEM preamplifier.
Induced Neutron Fission Fragment
Transporting a fission fragment out of the target material and into the gas chamber fission fragment trasport out of U-233
Gaseous Medium Physical Concepts
Apparatus
Detector Description
DAQ
Detector Operation and DAQ Setup
Data Analysis
Signals observed from Scope
QADC spectrum
PADC spectrum?
ToF measurements (Describe the IAC apparatus in the Apparatus section)
Conclusion
"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.
Recommendations
1- Testing the detector in a reactor of a high neutron fluence to study the detector stability and radiation hardness.
Using the ESEM fcor to test the quality of the the procedure for applying ED7100 paste
References
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