H Proposal Defense

Topic

The performance of a neutron sensitive gaseous detector based on thick gaseous electron multiplier preamplifiers.(THGEM)

Introduction

Gaseous detectors using Gas Electron Multipliers <ref name="Sauli1997">F. Sauli, et al, NIM A386, (1997) 531-534 </ref > are becoming common in nuclear and high energy physics experiments. The Gas Electron Multiplier acts as a preamplifier inside the detector allowing the detector's charge collection elements to decrease in size. A kapton based gas electron multiplier (GEM) </ref>requires the technology to etch an array of 50 micron diameter holes in 100 micron thick foils. Thick Gas Electron Multipliers (TGEMs) have 2 mm diameter holes which can be manufactured using a CNC machine. The objective of this work will be to measure the performance of a TGEM that is doped with Uranium to measure fast neutrons that produce fission fragments within the ionization chamber in the presence of the Uranium. This fission chamber like device will have the advantage of measuring the location of the incident neutron on the face of the detector.

Neutrons are nucleons with neutral charge, They interacts generally with high atomic number elements that have high cross sections for certain reactions.Mostly,the neutron interactions are represented by fission, capture,conversion (to an alpha or a proton) and scattering.

A neutron THGEM detector project is basically building the detector using the standards related to THGEM detectors, but adding to the detecting surface an element with a high cross section for a neutron interaction. In our case the THGEM foil will be covered by thorium-232 or uranium-238 which have relatively a high cross section for fission when the neutron energy range is between 5-14 MeV.

The THGEM neutron detector is supported by a read out wire net to get the signal, the read out wire net is connected to a gum stick and a break board, a software (coda) is responsible for collecting and filing the data, then analyzing the data by ROOT (software package).

The quality of the detected signal is dependent on many factors, but the study will be representing the effect of the thickness of the coated radioactive material for U-238 or Th-238 on the detector efficiency. OR comparing the detector properties and mainly the efficiency when it is coated by U-238 layer thickness with case when the coting is Th-232. Also reaching a design for THGEM card maximizes the gain with a the least sparking rate under a voltage applied up to 2 kV.

Chapter 1

Electron Multiplication

Electrons are fermions, negatively charged particles, usually bound to the nucleus of an atom. Ionization is the process by which electrons are liberated from the confines of an atom. The minimum amount of energy required to liberate the electron is referred to as the ionization energy, the electron becomes free when the energy transferred to it is greater than the ionization energy.

The passage of heavy ions through a gas is one way in which to transfer energy to the bound atoms of a gas and liberate them.

Specify the least amount of KE a fission fragment needs to ionize the electrons in an Argon atom.

Least fission fragment K.E needed for ionization

Lets have positively charged particle like a fission fragment with a certain kinetic energy in an argon gas, electron will starts scattering around the path in all direction for many reasons, but the starting point is bremsstrahlung radiation produced by the decelerating fission fragment,or by colliding directly with the gas atom.

Bremsstrahlung is not necessarily the initiator of ionization.

The primary electrons produced may also interact with the gas electrons and release more electrons through electron-electron interactions.

In the case of low interacting particles with the gas, the number of the electrons released will not be enough to give an indication of the existence of this particle, so any electron produced primarily should be multiplied.

The free electrons are collected by the surrounding electric field toward a foil with holes, as the electron passes through one of the holes it will be multiplied. <ref name="Sauli1997">F. Sauli, et al, NIM A386, (1997) 531-534 </ref >

pic referece <ref name="CMB_KEK"> CMB Group "Detector technology project connects fields", www.kek.jp/intra-e/feature, Dec.4 2010,http://www.kek.jp/intra-e/feature/2009/KEKDTP.html </ref >

Microscopically, as the electron is accelerated when it passes through a hole in which the electric field lines diverge strongly. The electrons increased kinetic energy becomes sufficient to ionize other electrons bound to the gaseous atoms. Electrons from this secondary ionization are then accelerated and become the initiators of further ionization events. This process is known as Gas Avalanche multiplication. <ref name="Shalem2005"> Shalem Chen Ken, "R&D of a novel gas electron multiplier – the THGEM", Master Thesis,the Scientific Council of the Weizmann Institute of Science </ref >

Gas Electron Multiplier (GEM)

The Gas Electron Multiplier (GEM) was invented by Sauli<ref name="Sauli1997"/> in 1997 using recent developments in lithography. The devise is made from "flex circuit" technology which is based on 50 micron thick kapton foils clad on both sides with 5 microns of copper. Lithography is used to etch a staggered pattern of micron diameter holes equally space by distances comparable to the hole diameter. The small size facilitates the use of low voltages (300 Volts) in order to generate the electric field for amplification. By comparison, the typical drift chamber, operating on the same principle, would need more than 1 kV to establish a similar electric field. The GEM foil is flexible enough to be be curved allowing cylindrically shaped ionization chambers with larger active areas.

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Nevertheless, GEM foil sparking rate increases rapidly,micrometer scale for the design make it hard to maintain. When the design of GEM detector becomes in millimeter scale, the characteristics will change, the new design is called THGEM (thick gaseous electron multiplier). With the new design for THGEM, the foil becomes rebust, has a higher operating voltage and relatively a higher gain.

Thick Gaseous Electron Multiplier (THGEM)

THGEM detector consists of a gas chamber, THGEM cards, a charge collector composed of strips, and a high voltage distribution circuit. A THGEM card is made from FR4/G10 clad on both sides with copper. The THGEM card is a 12x12 cm square plate that is 1 mm thick with 17 um thick copper cladding<ref name="G. Agócs"> G. Agócs,JINST 3 P02012 2008</ref>. Each card is chemically is etched to leave a copper trace around the perimeter of the card that is 10 cm on each side as shown in the figure XX. A thin layer ( 5 microns) of resistive paste (ED-7100) is applied to the card to permit current to flow on the surface. The card is then machined to have a hole of diameter 0.5. The resistance paste near the hole is machined away to form a 0.15 mm diameter rim around the hole. The holes are formed in a staggered array with a pitch of 0.8 mm.

Chapter 2

Neutron Sensitive THGEM

 Haitham: I think you should try learning how to construct sentences 
and paragraphs using the ISU thesis checking center.  
Take the paragraph below to them and ask them to check it.

Sensitivity is a scale for the detector capability to give a usable signal for certain particle(s)[6].Sensitivity for THGEM as a neutron detector is mainly depending on neutron energy range, material cross sections for ionization, neutrons rates of interactions,detector mass (in tons in the case of neutrinos),background, noise (sparking and electronic devices used to collect the signal) ,materials surrounding the detectors and the strength of the electric field.[6,7]

Assuming a given neutron with fixed energy and flux as well as a fixed material interaction thickness, then the choice of nuclide is the remaining variable to maximize the fission rate of the detector.

Getting the maximum efficiency requires maximizing the rate of interactions by an appropriate choice for neutron and ionization sensitive materials, supported by decreasing the noise and increasing the strength of the electric field in a detector mass built based on THGEM transparency offering a detection area changeable to adapt any application.

Fast neutron X-sect for U-238 and Th-232

The cross section is the proportionality constant for the relationship between the particle traveling distance dx and its probability to make an interaction.

The cross section values are represented as a function of energy. The importance of these curves is giving the value of the cross section for each energy and showing the resonance peaks.Theoretically, there is not any model that gives a detailed prediction of cross section curve, but statistically it is possible to evaluate the parameters for an assumption that describes part of the cross section curve within a certain error.

Neutron fission is one of the interactions commonly takes place spontaneously or under certain experimental conditions. An incident neutron moving with kinetic energy hits a nucleus to produce new nuclei (fragments) and particles.

Neutrons are classified depending on the their kinetic energy into: thermal, intermediate, and fast neutrons. the following table shows the range of each group. Also it shows additional types of neutrons that are important in applications that have neutron energy less than the intermediate range.

Type	Energy
Thermal	0.025 eV
Epithermal	0.025 eV – 0.2 eV
Resonance	1 eV – 1000 eV
Intermediate	1 keV – 500 keV
Fast	> 0.5 MeV

[Ref.4]

U-238 and Th-232 belong to the actinides. There are characterized with relatively high neutron fission cross section for fast neutrons (specifically for neutron energy higher than 1.5 MeV ).

Change the red and green colors to black.

Chapter 3

Expected Detector Properties

Th-232

The background energy spectrum for Th-232 has a main for alpha decay of energy 4082.8(14) keV combined with gamma rays of energy 63.81( 1) keV and intensity 26.3(13) percet.<ref name="Th-232"> http://home.fnal.gov/~hannahnp/decay/decay.html, Jan.5 2011,http://atom.kaeri.re.kr/cgi-bin/nuclide?nuc=Th-232</ref >.

.<ref name="Th-232"> http://home.fnal.gov/~hannahnp/decay/decay.html, Jan.6 2011,http://atom.kaeri.re.kr/cgi-bin/decay?Th-232%20A

<ref name="U-238"> Fermilab,http://home.fnal.gov/~hannahnp/decay/decay.html, Jan.5 2011,http://home.fnal.gov/~hannahnp/decay/U238.html</ref >

<ref name="Th-232"/>

Signal Size

The signal of the detector appears as result of the electron multiplication which is represented by the gain. The effect of the ions and fission fragments on the signal is negligible. The previous result can be proved mathematically or by the simulation.

Not only do the free electrons share in the signal, but also the electronics used to detect the signal does, the figure shows a fast single photon pulse combined with another two electronics noise peaks <ref name="Chenchik2004"> R.Chechik,A.Breskin,C.Shalem, NIM A535, (2004) 303-308 </ref > .

Furthermore, the signal reflect the magnitude of gain obtained as shown in the figure which represents a 60 keV X-ray signal when the gain magnitude is 180 and 220 respectively.<ref name="Bondar"> Bondar,A.Buzulutskov,A.Grebernuk, aXiv:0805.2018 </ref >

Fig. Double THGEM signal with electronic noise<ref name="Chenchik2004"/>

Fig. Double Fig.Signal produced when the gain is 220 and 180 respectively<ref name="Bondar"/>

The expected signal from the designed detector is composed of electron multiplication peak,background peak(s) of the radioactive isotope used (even U-38 or Th-232) and electronics noise peaks.

Efficiency

Efficiency requires maximizing the rate of interactions by an appropriate choice for neutron fission sensitive material, fission occurs and fission fragments will be created in a gas chamber with a known ionization rate under a certain pressure in a highly dense electric field environment. Amplification occurs for the number of electrons in three stages.The electrons will hit the read out card producing a signal collected by data acquisition system (daq) within a certain electronic noise.

The in our case, the efforts are going to be mostly spent to increase the detector efficiency by studying the radioactive material layer thickness for U-238 or Th-232 to get the highest fission rate.Or comparing the efficiency of the detector when it coated by a layer of U-238 with the one that will be coated with Th-232 layer. In addition to get the best THGEM card design to reach the highest gain with the least sparking rate under a high voltage which may reach up to 2kV.

References

<references/>

1- Media:Shalem_MSthesis_march2005.pdf

2- http://www.kek.jp/intra-e/feature/2009/KEKDTP.html

3- http://wiki.iac.isu.edu/index.php/Data_Acquisition

4- General principles of neutron activation analysis, J. Dostal and C. Elson,p 28 Figure 2.3.

5- Paul Reuss ,Neutron physics,L'editeur EDP Sciences,2008.

6- William R. Leo,Techniques for nuclear and particle physics experiments,1st edition,Springer Verlag, 1995.

7-Ch. Jammes, P. Filliatre, B. Geslot, L. Oriol, F. Berhouet, J-F. Villard, “Research activities in fission chamber modeling in support of the nuclear energy industry”, ANIMMA International Conference, 7-10 June 2009, Marseille, France.

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