Difference between revisions of "Performance of THGEM as a Neutron Detector"

From New IAC Wiki
Jump to navigation Jump to search
 
(228 intermediate revisions by 2 users not shown)
Line 1: Line 1:
 
=Title=
 
=Title=
  
−
The Performance of Thick Gaseous Electron Multiplier (THGEM) Preamplifiers as a Neutron Sensitive Detector.
+
The Performance of a Fission Chamber equipped with Gaseous Electron Multiplier (GEM) Preamplifiers.
  
−
=Introduction=
+
=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.
  
  
−
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.
+
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.
  
−
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.
+
/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.
  
−
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.
+
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 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.
+
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.
  
−
=Chapter One : Ionization=
+
=Introduction=
  
−
Ionization is the liberation of an electron from the confines of an atom.  The minimum amount of energy required to liberate the electron is referred to as the ionization energy.  Energy transferred to the electron in excess of this ionization energy will appear in the form of the ejected electron's kinetic energy. Photons or charged particles like fission fragments interacting with a gas volume can induce ionization. The ionization process  depends stochastically  on the ionization cross section which is mainly affected by the fission fragment energy, type of the fission fragment (heavy or light), the gas pressure in the chamber, and the atomic properties of the gas.  Generally, the amount of energy needed to have an ionization event in a gas is the same on average  <ref name="Veenhof"> R. Veenhof, Internal Note/TPC, ALICE-INT-2003-29 version 1.0, 2003</ref>, regardless of the incident particle type or energy as shown in the following table for argon gas.
 
  
−
{| border="1" cellpadding="4"
+
why do we want neutron detector
−
|-
 
−
|Type of particle and its energy || 9 keV x-rays || 10 keV electrons || 40 keV electrons || x-rays Ar-37(K-capture)(5-25 keV) + beta ||alpha 7.68 MeV || 340 MeV protons
 
−
|-
 
−
|Energy per ion-electron pair (eV) || 27.9 <math>\pm</math> 1.5 ||    27.3            || 25.4    ||  27.0 <math>\pm</math>0.5 || 26.25 || 25.5
 
−
|}
 
  
 +
types of neutron detectors
  
−
Ionization by fission fragment is not  the only source for a signal, but it is not the only ionization process, there are other  physical processes occurs in the medium which are related to the gas mixture properties or the preamplifier material properties like: Photoionization, thermal ionization, deionization by attachment (negative ion formation) , photoelectric emission, electron emission by excited atoms or positive ion,and field emission <ref name="Kuffel"/>.
+
gaseous neutron detectors
  
−
Choosing 90/10 Ar/CO2 gas mixture was not random. The behavior of the mixture is studied  based on simulating the ionization in the gas mixture as it is in a gas chamber that has a wire cathode and an anode to produce a high electric field<ref name="Veenhof"/>. Garfield and  Magboltz software packages were used for this purpose, Also Imonte 4.5 program can be  for more precise simulation but it was not used for the simulations below <ref name="Veenhof"/>.
+
fission chambers
−
The results of 90/10 ArCO2 simulation is summarized by the following:
 
  
−
===Why 90/10 Ar/CO2 Mixture (Simulation by Garfield) <ref name="Veenhof"/>===
+
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.
  
−
* Drift velocity: Increasing the percentage of CO2 the gas mixture at low electric field increases the drift velocity as shown in the figure:
+
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.
  
−
[[File:drift_velocity_percentage_CO2_inAr.png || 100px]]
+
{| border="1" cellpadding="5" cellspacing="0" align="center"
 +
|-
 +
|  Medium Phase || Signal Generation by || Material Structure  || Neutron Energy || Notes 
 +
|-
 +
| rowspan="2"  |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 drift velocity is saturated but at different values depending the percentage of the CO2, so the increase in the drift velocity becomes smaller withe increase in CO2 percentage int he gas mixture as show in the figures below:
+
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:
  
−
[[File:max_drift_velocity_CO2percent_Efieldvalue.png || 100px]]          [[File:Vmax_CO2percent.png || 100px]]
+
[[File:MPGD_types.jpg | 600px]]
 +
 
  
  
−
*Gain: It is dependent on Townsend coefficient, the figure below shows the gain-Electric field relationship for different mixtures of ArCO2, in addition to the effect penning of the gain.
+
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.  
  
−
[[File:Townsend_coeffiecient_inArCO2.png || 100px]]
+
{| border="1" cellpadding="4"
 +
|-
 +
| 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) ||[http://arxiv.org/ftp/physics/papers/0408/0408074.pdf]
 +
|-
 +
|F. Murtas || Fast ||  60 mm polyethylene || 2-20 || ~1.0 || <math> 10^{-4} </math> at 60 keV || low photon sensitivity <math> 10^{-8} </math>  [http://iopscience.iop.org/1748-0221/7/07/P07021/pdf/1748-0221_7_07_P07021.pdf]
 +
|-
 +
|M. Shoji || thermal  || B-10 || 0.025 || 1.2 || 0.027 || [http://iopscience.iop.org/1748-0221/7/05/C05003/pdf/1748-0221_7_05_C05003.pdf]
 +
|-
 +
|}
  
−
The figure above is not accurate enough since Garfield and Magboltz software packages use the first two terms of  Legendre polynomials as solution for Boltzmann transport equation (Magboltz may extend to the third term when the simulation goes in the longer time one), but using using IMonte 4.5 uses the spherical harmonics which improved the simulation and make independent of the expansions that describe the electron energy distribution. <ref name="Biagi"> S.Biagi Nucl. Instrum. Methods, vol. A 421, pp. 234–240, 1999</ref >
+
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.
  
−
The effect of Penning on Townsend coefficient is represented by the following :
 
  
−
[[File:Townsend_coefficient_inArCO2_Penning.png || 100px]]
 
  
−
Comparing Ne/CO2 with Ar/CO2 considering the 40 percent of Penning transfer:
 
  
−
[[File:gain_NeCO2_ArCO2_full_40percent_penning_transfer.png || 100px]]
 
  
−
*Ionization rate:
 
  
−
[[File:ArCO2_ionization_rate.png || 100px]]
 
  
−
*Attachment:
+
==Motivation==
  
−
[[File:ArCO2_attachment_Efieldvalue.png || 100px]]    [[File:ArCO2_attachment_losspercent_anodevoltage.png || 100px]]
+
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==
  
−
===Main differences between electrons and ions behavior in a gas<ref name="Mason">Mason, Edward A. and Earl W. MacDaniel. 1988. Transport Properties of Ions in Gases. John Wiley & Sons. </ref>===
+
Decrease time needed to detect fission fragment. (increase the cathode voltage)
  
  
−
There are main differences in behavior regards the diffusion between ions and electrons in a gas in the existence of the electric field. The ratio between the mass of the electrons and the gas atoms is very small, so with a few eV work done by the electric field, the electrons will gain a high velocity compared to that of the ions when are accelerated under the same electric field.
+
Determine the pulse length of a signal from the GEM detector and compare to a fission chamber.
  
−
The probability of low energy electrons to make an interaction is higher that of the low energy ions supported by accurate calculations for the electron drift velocity. Electrons at low energy have the ability to produce vibrations and excitations  in the gas atoms or molecules measured within the lab frame, but the low energy ions have a very low cross sections for the most of the interactions with the gas atoms or molecules, in addition to complexity in measuring the products of these interactions. Furthermore, the ratio between a gas atom or molecule is very small which simplify the calculations for the velocity distribution for the electrons in many gases.
 
  
−
Producing the electrons in a gas is simpler than producing ions in the same gas. a Large number of interactions appear in the gas for producing electrons like thermionic emission, photoemission, or a radioactive decay, but  creating an ion requires electron bombardment, photo-ionization or an electric discharge which require more sophisticated conditions for the experiment , especially that the ions are not as sensitive as the electrons for the the non-uniformity of the electric field, electric potential and magnetic field.
+
Rad hard pre amplification.
  
−
The last difference that might be a concern in our case is the existence of the impurities. The electrons loses most of their energy in the molecular level compared to the electron energy loss  within the atomic level for a pure gas. On the other hand, the ionic velocity distribution is not affected by the existence of these impurities except for some cases related to a highly accurate ionic studies in gases.
 
  
−
=== Gas Quenching===
+
Detecting neutrons with E > 1 MeV?
  
−
  Rewrite the first two sentences so quenching is more clearly described.
+
=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.  
  
−
Gas quenching is a non-ionizing process occurs when a gas molecules with large cross sections for excitation and vibration states decreases 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 which 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"/>:
 
  
  
−
{| border="1" cellpadding="4"
 
−
|-
 
−
|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.
+
[[ Induced Neutron Fission Fragment]]
  
−
==Townsend's Coefficients==
+
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]]
  
−
===Townsend's First Coefficient===
 
  
−
Townsend's first Coefficient is defined as "the number of produced by an electron per unit length in the direction of the electric field"<ref name="Kuffel"/>.
+
[[GEM pre-amplification]]
  
 +
=Apparatus=
 +
==Detector Description==
  
−
*Townsend started his investigations about discharge after fundamental studies  were known around 1899 about:
+
[[Detector Description]]
  
−
1- Conductivity  production by x-rays.
+
==DAQ==
  
−
2- Diffusion coefficients, mobility of ions and ion-electron
+
[[Detector Operation and DAQ Setup]]
−
recombinations.
 
  
−
* It was observed for an increment in  Electric filed E and pressure P beyond the saturation current value, at some critical value of E and p, the current increases rapidly which will lead to a breakdown of the gap in the form of a spark  <ref name="loeb"> L.B.Loeb, basics processes of gaseous electronics, University of California Press, 2nd edition, 1955. </ref>.
 
  
−
*Townsend studied the relationship between E/p as a function of x, where x is the separation distance between the plates. His study was based on the photoelectron emission from the cathode by ultraviolet light at high uniform electric field up to 30kV/cm and 1 atm pressure.
+
[[DAQ]]
  
 +
=Simulation and Analysis=
  
−
*He plotted different values for E/p, he found that the slope of the line is <math>\alpha</math> which is "the number of the new electrons created by  single electron in 1 cm path in the filed direction in a gas at appropriately high E/p" <ref name="loeb"/> . Townsend plotted <math> \frac{i}{i_0} </math> against the distance of separation x, he concluded the following equation
+
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.
  
−
<math> ln (\frac{i}{i_o}) = \alpha x </math>
+
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.
  
−
to calculate <math>\alpha</math> (the slope)  for different values of E/p as shown in the figure:
+
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.
  
  
−
[[Image:alpha_town1st_coff_1.png |thumb| Fig. relationship between <math>ln{i}/{i_o}</math> and the distance x for different values of E/p <ref name="loeb"/>]]
+
===Alpha, electrons, photons ionization ===
  
−
* Townsend studied  <math>\alpha</math> as a function of E/p for a given gas, he founded ,for different values of p, 
+
[[Alpha Ionization]]
−
<math>\alpha</math> experimentally is different from expected value calculated, but the plots met in values when it represented the relationship between  <math>\alpha</math>/p as a function of E/p as shwon in the figure below.
 
  
−
[[Image:alpha_p_E_p_rations.png |thumb| Fig. shows alpha p ratio coincides with E p ratio <ref name="loeb"/>]]
+
[[Beta Transmission  and Ionization]]
  
 +
[[Gamma Emission]]
  
−
*The relationship between <math>\alpha</math>/p and E/p is shown below.  However, the equation can not predict all the values of <math>\alpha</math>/p accurately for different values  of E/p, i.e having a single analytical function to fit the experimental results for a gas does not exist, because <math>\alpha</math>/p is dependent on the number of electrons produced  which changes as the average energy distribution of the ionizing electrons changes <ref name="loeb"/>.
+
=Data Analysis=
  
−
<math> \frac {\alpha}{P}= Ae^{(\frac{-BP}{E})} </math>
+
Signals observed from Scope
  
−
;Theoretical evaluation of <math>\alpha/p</math> as a function of E/p
+
[[QADC spectrum]]
  
−
* The first attempt was done by Townsend when the experimental data were limited by the to the high E/p.
 
  
−
===Decreasing the discharge in THGEM===
+
PADC spectrum?
  
  
−
THEGM preamplifier is designed to be rebust, economical, and to get the maximum gain with the least discharge effect.
+
ToF measurements (Describe the IAC apparatus in the Apparatus section)
  
−
The discharge effect is when you experimentally start observing sparks coming from the detector. Whenever discharge becomes phenomenon to study then the probability of discharge is used. The probability of discharge is defined as the ratio between the observed frequency of the breakdown and source rate <ref name="bachmann"/>.The discharge rate and the source rate can be represented as function of position as shown in the figure.
+
=Conclusion=
  
−
                             
+
"Neutron detectors are used in several different applications and are of great
−
[[Image:sourcerate_dischargerate_position_bachmann.png |thumb| Fig. Discharge rate and the detected source rate can be represented as function of position <ref name="bachmann"/>]]
+
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.
  
−
Producing these sparks refers to many reasons,it is obviously observed  when a highly ionizing ion passes through the gaseous chamber and produces enough free electrons to break down the rigidity of surrounding gas by having an avalanche size exceeds Raether limit ( <math> 10^7</math> electron-ion pairs) when separating the electrodes vertically with small a distance <ref name="bachmann"> Bachmann et al NIM A 479 (2002) 294-308 </ref > .
+
Homeland security application and a need for a large detection area.
  
−
Temperature, humidity, and gas flow externally affect the probability of the transition from the proportional multiplication to a discharge at a given potential, the effect clearly appears in absence of the amplification internal effects as the design quality and the history of the electrodes <ref name="bachmann"/>.
+
A need for detectors that has the ability to discriminate gamma radiation.
  
−
In case of heavily ionizing ions like alpha particles, an increase in gain causes the probability of discharge to increase, but the increment in the probability of discharge can be decreased by choosing an appropriate gap between the THGEM cards <ref name="bachmann"/>. As a result, achieving a maximum gain for an incident particle on a chamber with a specific gas mixture ,under a  voltage applied on the THGEM cards, requires an appropriate distance that increases with increment of the ionization rate, i.e an alpha particle requires a bigger gap between the THGEM cards than that of a gamma ray to avoid the discharge effect.(can be experimentally proven).  
+
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.
  
−
[[Image:Discharge_probability_gain_doubleGEM_bachmann.png |thumb| Fig. Discharge probability as function of gain for double GEM detector <ref name="bachmann"/>]]
 
  
−
=Boundary Element Method (BEM)=
+
==Using the ESEM fcor to test the quality of the the procedure for applying ED7100 paste==
  
−
Boundary Element Method is used to solve Laplace or Poisson Equation, a function u(x,y,z) is solved on the domain boundary and the function partial derivatives are evaluated by integrating on the number of elements on the boundary.<ref name="Kuffel">  Kuffel, W. S. Zaengl, J. Kuffel, High voltage engineering: fundamentals, Biddle Ltd, 2nd edition, 2000 </ref>.
+
[[procedure and results]]
  
−
=References=
 
  
 +
== Thesis Adds ==
  
 
{| border="1" cellpadding="4"
 
{| border="1" cellpadding="4"
 
|-
 
|-
−
|physical parameter || Effect on the detector properties <ref name="Veenhof"/>
+
| section || suggestions || Notes
 
|-
 
|-
−
||Electron drift velocity || Dead time
+
| 2.2.1    || adding figures U-238 and Th-232 xsection figures. ||
 
|-
 
|-
−
| Electron transverse diffusion || Spatial resolution (momentum resolution), transverse resolution should match the response function (signal width)
+
| styling || Sections, Tables, equation Numbers ||
 
|-
 
|-
−
| Townsend Coefficient || Gain which improves the resolution
+
|Theory || add theoretical view for detector signal analysis?
 
|-
 
|-
−
|Attachment Coefficient || Losing the information about an ionization, affects the position information and dE/dx identification
+
|2.5.1 || bethe-bloch plot for 90/10 Ar/CO2
 
|-
 
|-
−
|Gas breakdown || Discharge at that voltage
+
|2.6 || adding a section for ion diffusion.
−
|-
+
 
−
| Ion mobility || Determine the rate of collecting the electrons (if the space charge is eliminated), the signal duration in the readout plate
+
 
−
|-
 
−
|Ionization rate || Affect the spatial resolution, dE/dx identification
 
 
|}
 
|}
 +
 +
=References=
 +
 +
  
  
 
<references/>
 
<references/>

Latest revision as of 20:52, 9 April 2015

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:

MPGD types.jpg


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

<references/>

Retrieved from "https://wiki.iac.isu.edu/index.php?title=Performance_of_THGEM_as_a_Neutron_Detector&oldid=100258"