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/[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 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.

GEM detector has been designed, developed and used for detection in CERN since 1997. Fabio Sauli invented the GEM preamplifier in 1997<ref name="Sauli1997">F. Sauli, et al, NIM A386, (1997) 531-534 </ref > and Gandi and De Oliveira designed it. The deign was on 50,5 um kipton copper clad cards, they had holes of 70 um in diameter in a an equilateral triangular pattern with a 140 um pitch distance.

/may use

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 <ref name="Agocs">G. Agocs, B. Clark, P. Martinego, R. Oliveira, V. Peskov,gand P. Picchi,JINST, 3, P020112, 2008 </ref >. 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.

Such a high multiplication made them highy desirable to use for complicated experiments such as compass (2007) and TOTEM (2008) for tracking and triggering.

The objective of this work is to construct a GEM based ionization chamber. The detector will be made sensitive to neutrons by coating the cathode with a fissionable material. The cathode will take a further distance from the first GEM preamplifier than the one used for the original GEM detector design. 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

why do we want neutron detector

types of neutron detectors

gaseous neutron detectors

fission chambers

Neutron detectors have many applications in nuclear, medical, and industrial applications. Researchers explore new theories and phenomena that are related to many applications in different fields. They collect the data from the detector, and analyze it to evaluate the detector characteristics to use the detector in a specific application. For instance, using the neutrons in imaging; the detector with higher efficiency and better spatial resolution gets the best image. As an application, Fast neutrons are also used for scanning cargo containers: since they are highly penetrating particles, fast neutron detectors can be used for counting and imaging to check the contents of the containers. Also, neutron detectors are essentially used in nuclear reactors, since they are important in determining the reactors’ stability during operation, and in the radiation monitoring in the area surrounding the reactor.

They are many types of neutron detectors because of the wide variety of applications; they can be classified depending on the type of neutron interaction, the type of medium that is responsible for producing the signal, or the energy range of detected neutrons. They can also be classified based on their performance which is determined by calculating their efficiency, energy resolution, sensitivity to gamma particles, detector dead time, and spatial resolution. The following table shows some of the neutron detectors and their characteristics.

Medium Phase	Signal Generation by	Material Structure	Neutron Energy	Notes
Solid	scintillation	Plastic	10-170 MeV	Its length 2m length with a PMT, dopant may decrease the energy range.
	scintillation	Inorganic, LiBaF3:Ce	Range is dependent on the dopant	Ability to distiguish heavy charged particles.
Liquid	scintillation	BC501A	0.004- 8.00 MeV	Liquid scintilators are rarely used, efficiency 1-3 %
Gaseous	Ionization	Ar-CO2(70/30)and B-10	< 1 MeV	B-10 is doped on each GEM foil, efficiency may reach to 30% for thermal neutrons

The gaseous detector have passed through stages of development to improve signal's width and amplitude. Gaseous detectors started with a gas chamber that contained only a capacitor, a high voltage was applied to collect the ions and electrons after ionization. The disadvantage was the signal of order of milliseconds which is very slow for most applications. So the next step was to make the pulse width shorter; Rather than waiting for the electrons to reach the anode, a grid was placed between the cathode and the anode to shorten the distance crossed by the electrons, as result, the pulse width was improved to the microsecond scale, the new detector had been called Frisch grid detector. Recently, the detectors are developed to be particle counters, time projectors, or used for imaging purposes, so proportional counters, wire chambers, and micro-pattern gaseous detectors (MPGD)were developed to satisfy the demand. The MPGD detectors is almost the latest the improvement in gaseous detectors, they are many types and patterns for these detectors, they helped to detect the lower ionizing particles followed by electron multiplication by the electric field, such process improved the signal's width to be of the order of nanoseconds. the following figure shows different patterns of MPGD detectors:

Building a neutron detector based on GEM electrodes has been done for imaging purposes. They are few groups who were interested in constructing neutron detectors for imaging using a neutron flux. Most of detectors had a neutron converter based on polyethylene for fast neutrons of energy (1-20 MeV), or B-10 coating for detecting thermal neutrons. The following table shows the properties of the previously built neutron detectors.

Author	Neutron Type	Converter	n-energy (MeV)	FWHM (mm)	efficeincy	Notes
dangendorft	Fast	polypropylene n->p	2-10	0.5-1.0	0.05%( 2 MeV) and 0.02% (7.5 MeV)	[1]
F. Murtas	Fast	60 mm polyethylene	2-20	~1.0	[math] 10^{-4} [/math] at 60 keV	low photon sensitivity [math] 10^{-8} [/math] [2]
M. Shoji	thermal	B-10	0.025	1.2	0.027	[3]

All the previous trials meet in low efficiency for detecting the fast and the thermal neutrons. The aim of work is to study and build a neutron detector with a higher efficiency for thermal and fast neutrons by installing U-233 thin film, as a fissionable material with a high fission cross section to be a neutron sensitive material, that is coated thin film on the cathode surface inside a gaseous chamber, which has three GEM preamplifiers.

Motivation

Fast neutron detectors have many applications in different disciplines of nuclear technology. For instance; 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 surface 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, it will exchange charge with the medium's atoms and molecules until it becomes neutral, meanwhile electrons will primarily scatter and they mostly diffuse in the direction of drift electric field which guide them to the first GEM preamplifier. If the electron passed through one of the GEM's preamplifier holes, it will accelerate and ionize the surrounding atoms, that will cause electron multiplication. Then the number of electron increases creating avalanches then streams. The electrons ended their trip as they had been collected by charge collector to create a negative pulse on the oscilloscope display.

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

GEM pre-amplification

Apparatus

Detector Description

Detector Description

DAQ

Detector Operation and DAQ Setup

DAQ

Simulation and Analysis

The interactions of \alpha, \beta, \gamma, and fission fragments within the gas volume of the GEM detector were simulated using a Monte Carlo program known as GEANT4 [insert G4 reference] to interpret the detector's signal. Chapter X describes the theory of the interaction that may occur. of this work discussed a number of important theories that justify the physical processes which occurred in the detector, such as induced neutron fission, ionization, diffusion. Using the appropriate software, their simulations are important to have an accurate analysis for the detector performance, when the simulations consider all the the experimental constraints, they lead to a good assessment for the experimental measurements.

Installing U-233 to build a fission chamber allows more than one type of particles to charge the QDC. U-233 coating emits alpha, beta or gamma particles, they travel through the drift region with a specific energy, those particles loss energy which mostly leads to gas ionization. The same case for the detector exposure to a neutron flux, a fission fragment passes through the drift region, and ionizes the gas. The QDC charge spectrum may represent the ionization of either one or more than one particle without specifying the source of the charge, so it is important to study the ionization simulation to distinguish the estimated charge for each particle.

The aim of this chapter is to present the simulation analysis that estimates the initial charge of each particle that is deposited in the gas, with and without the effect of an FR4 shutter. When an ionizing particle or a fission fragment passes through the drift region, there is an effect on their initial charge depending on the detector's shutter position. As mentioned previously, the detector modified structure has a shutter that is used to cover U-233 coating area, so it stops (or partially stops) all the positively charged ions when it is closed, and allows those ions to pass through the gas when it is open. The next sections will discuss the steps for simulating the ionization to estimate the magnitude of the initial charge to compare it to the QDC charge spectrum measurements.

Alpha, electrons, photons ionization

Alpha Ionization

Beta Transmission and Ionization

Gamma Emission

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

procedure and results

Thesis Adds

section	suggestions	Notes
2.2.1	adding figures U-238 and Th-232 xsection figures.
styling	Sections, Tables, equation Numbers
Theory	add theoretical view for detector signal analysis?
2.5.1	bethe-bloch plot for 90/10 Ar/CO2
2.6	adding a section for ion diffusion.

References

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