Difference between revisions of "H Proposal Defense"
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Finally, when the fission fragment is totally neutralized (surrounded by its all electrons), the kinetic energy is very low to make any ionization, In this case the fission fragment minimum kinetic energy is represented by the energy of the last attached electron which is between 15.7 eV and 3.2 keV. | Finally, when the fission fragment is totally neutralized (surrounded by its all electrons), the kinetic energy is very low to make any ionization, In this case the fission fragment minimum kinetic energy is represented by the energy of the last attached electron which is between 15.7 eV and 3.2 keV. | ||
− | The ionization process is dependent on other factors based on the medium properties. The ionization is a stochastic process. The probability to have a collision between the fission fragment and an argon gas atom is mainly dependent on the the fission fragment energy, heavy or light fission fragment, and the gas pressure in the chamber. Also | + | The ionization process is dependent on other factors based on the medium properties. The ionization is a stochastic process. The probability to have a collision between the fission fragment and an argon gas atom is mainly dependent on the the fission fragment energy, heavy or light fission fragment, and the gas pressure in the chamber. Also the atomic properties of the gas affect the process. In case of argon gas, a escaping peak effect is observed which decreases the number of the emitted electron by exciting the argon atom to emit a escaping k-shell x-rays photons from the surrounding medium. |
==Electron Multiplication== | ==Electron Multiplication== |
Revision as of 17:16, 30 March 2011
Topic
The Performance of Thick Gaseous Electron Multiplier Preamplifiers (THGEM) as a Neutron Sensitive Detector.
Introduction
What is main point of the paragraph. 1st sentence in paragraph introduces the main point. 2-4 sentences supporting the main point. 3 - 5 summarized the point
I propose to construct and measure the performance of a fission chamber which has been instrumented with preamplifiers known a Thick Gas Electron Multipliers (THGEM). This Fission chamber is a gas chamber filled with an inert gas enclosing a fissionable target material, like Uranium or Thorium. The fissionable material will produce fission fragments when an incident neutron of sufficient energy impacts material. The fission fragments which are within 5 microns 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 be amplified by the THGEM preamplifier and transported to charge collectors using strong electric fields.
A THGEM preamplifier is a perforated fiberglass board (PC board). 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 microns of copper. The copper clad kapton is perforated with 50-100 micron diameter holes in a staggered array separated by 100-200 microns. The THGEM preamplifier is more macroscopic using 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 in the THGEM , the fiberglass. 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 THGEM design) 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 shown to work. This fission chamber-like device will have the advantage of measuring the location of the incident neutrons on the face of the detector using a segmented charge collector.
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Chapter 1
Fission Fragment Ionization of Argon
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; when the energy transferred to the atom is greater than the ionization energy, the difference in energy will transfer to the electron as a kinetic energy. Ionization occurs when an energy carrier passes through gas, like phonons, photons,and charged particles, the amount of ionization is basically dependent on the energy loss of these energy carriers inside the gas.
Least fission fragment K.E needed for ionization
Bohr criteria for min fission fragment KE needed to ionize the Argon Gas.
Argon have binding energies between 15.7 eV and 3.2 keV.
https://wiki.iac.isu.edu/index.php/ArCO2_IonizationPhysics
The electrons are fermions. they are negatively charged particles with spin of half, usually bounded to the nucleus of an atom. Electrons can be generated by photoelectric, Compton scattering or pair production. When an external electric field is applied, the electrons moves with the direction of the electric filed lines from the negatively high electric potential point to the low electric potential one.
For charged particles passage with a certain kinetic energy through a gas, electrons will start scattering around the path in all directions because of the charge particles' collisions directly with the gas atoms, or by bremsstrahlung radiation produced by the decelerating charged particles when they are surrounded by a high electric field. Two kinds of electrons are freely moving in the gas chamber, the primary electrons generated by the incident particles, and the secondary electrons which are produced by electron-electron interactions.
Fission fragments is a source of ionization when they interact with the gas atoms. They loss their high energies in short range which make it hard to undertake in accurate details. Theoretically, the process can astochastically be described by dividing the fission fragment trip through the gas into three stages:
Firstly, as the fission fragment is totally ionized (without any bounded electrons), the total energy of is represented by :
where Z1 M,v are atomic number, the mass and the velocity of the fission fragment directly after the fission reaction. Coulomb force is including the repulsion force between the ionized gas atoms and fission fragment, in addition to, the attraction force between the fission fragment and electrons. But in at this moment the first term is dominant,since the fission fragment collision is causes the electrons to scatter away from the fission fragment.
Secondly, when the fission fragment decelerates, a charge exchange between the fission fragment and gas atoms, the electrons are even scattering away from the nucleus or attached to the nucleus as it is moving. The total energy loss of the fission fragment suggested by Bohr in 1940 is :
This formula is a first trial to evaluate the total energy loss by the fission fragment under each stage of charge carrier exchange between the fission fragment and the gas atoms, it is based on how accurate is the value of Z-effective at each stage, this will cause the first term to be dominant by the beginning of the motion of the fission fragment, but the second term is bigger close to the end of the fission fragment track.
Finally, when the fission fragment is totally neutralized (surrounded by its all electrons), the kinetic energy is very low to make any ionization, In this case the fission fragment minimum kinetic energy is represented by the energy of the last attached electron which is between 15.7 eV and 3.2 keV.
The ionization process is dependent on other factors based on the medium properties. The ionization is a stochastic process. The probability to have a collision between the fission fragment and an argon gas atom is mainly dependent on the the fission fragment energy, heavy or light fission fragment, and the gas pressure in the chamber. Also the atomic properties of the gas affect the process. In case of argon gas, a escaping peak effect is observed which decreases the number of the emitted electron by exciting the argon atom to emit a escaping k-shell x-rays photons from the surrounding medium.
Electron Multiplication
Electron multiplication is important in the case of the low interacting particles with surrounding medium, a small number of electrons released by the incident particle interactions or by the followed electron-electron interactions which is not enough to give an indication of the particle existence. So the primary and the secondary electron produced should have away to multiply by a preamplifier(like THGEM) to give an informative signal about the particle existence.
The THGEM preamplifier multiplication is based on the card sign. The card is deigned to have a high voltage difference between the top and the bottom side; an electric field will appear with its lines more concentrated in the drilled holes. As free electrons are produced, they are going to be collected by the surrounding electric field toward the card holes. As the electron passes through one of the holes, it will be multiplied, then the multiplied electrons may get multiplied again if the detector is supported with an additional card or more of the THGEM cards. <ref name="Sauli1997">F. Sauli, et al, NIM A386, (1997) 531-534 </ref >
![](/./images/thumb/4/4f/Hole_electric_filed.png/300px-Hole_electric_filed.png.jpeg)
Microscopically, a Gas Avalanche Multiplication process takes place in the preamplifier's holes. The electron is accelerated when it passes through the hole in which the electric field lines diverge strongly, The electron 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 as shown in the figure above.
Gas Electron Multiplier (GEM)
The Gas Electron Multiplier (GEM) was invented by Sauli<ref name="Sauli1997"/> in 1997 using recent developments in lithography. The device 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 spaced 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 curved, allowing cylindrically shaped ionization chambers with larger active areas.
Using GEM has advantages and disadvantages. GEM has a micrometer scale for the design that makes it hard to maintain, also GEM preamplifier sparking gain is rate increases rapidly as the voltage increases from 525V to 600V regardless of the pattern geometry, as a result, a operating voltage range decreases to about 300-350 V <ref name="A.Bressan1999">A.Bressan, et al, NIM A 424 (1999) 321—342</ref >. GEM has a low noise signal because of the relatively low operating voltage, also the micrometer scale hole diameter increases the electric field flux through the holes, so a relatively high gain is obtained in the range of the operating voltage.
When the GEM card is designed in millimeter scale; the new design is called the THGEM preamplifier. With the new design for the THGEM card, it becomes robust with a higher operating voltage and relatively higher gain, but the detector may have a higher noise compared to that of the GEM preamplifier.
Thick Gaseous Electron Multiplier(THGEM) Detector Design
The THGEM detector consists of a gas chamber, THGEM cards, a charge collector, and a high voltage distribution circuit.
The main part of the detector is the THGEM preamplifier. 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 a 1 mm thick coated with 17 um thick copper cladding.<ref name="G. Agócs"> G. Agócs,JINST 3 P02012 2008</ref> Each card is chemically etched to leave a copper trace around the perimeter of the card to be a border for a square area of a side length of 10 cm 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. Then the card is machined to have a 0.5 mm diameter hole. The resistive paste near the hole is machined away to form a 0.2 mm diameter rim around the hole. The holes are formed in a staggered array with a pitch of 0.8 mm.
The THGEM cards are fixed in place to have transparency through the holes using holders and separators in each corner, the cards are separated from each other by a vertical distance of approximately 2.6 mm.
A drift voltage card made of copper paper is placed at the top of the cards with a distance of 2.6 - 4 mm from the last THGEM card.
THGEM cards are inside a chamber made of two pieces of machined ertalyte (plastic) ; with a kapton window on the upper piece as shown in the figure. The two plastic parts are collected together by a number of M3 plastic screws located around the detector window to form a well close cavity around the THGEM cards away from the surrounding atmosphere. A 90/10 percent Ar/CO2 gas is pumped into the cavity with a pressure of 1 atm to generate electrons as the ionizing particles pass through it.
A read out plate exists on the lower side of the detector. It has a separation distance of 0.5 mm from the closest THGEM card, it is connected to 16 connectors, each connector has 20 traces. The connectors are then connected to a 130 pin connector by using a VFAT card which transfer the digital signal to a break board, then to the DAQ system.
Chapter 2
The Detector Physical Properties
Ionization and Calorimetry
Ionization rate is the number of the electrons released from the gas atoms in a certain period of time. Generally, the amount of the energy needed to have an ionization event in a gas is on average the same, regardless of the incident particle type or energy as shown in the following table in case of argon gas.
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 +_ 1.5 | 27.3 | 25.4 | 27.0 +_ 0.5 | 26.25 | 25.5 |
Fission fragments are also a source of ionization when they penetrate an argon gas chamber, the energy used for such an event is highly dependent on the fission fragment mass and velocity. When a fragment has low velocity, recoiling is competitive process to decrease the probability of ionization. In the case of a high velocity fragment, the probability of ionizing the gas increases to be closer to that of the alpha particles as they are penetrating the same medium.
In more specific studies that are based on classifying the fission fragments into light and heavy fission fragments; they tried to measure \delta E (ionization defect), the degree to which the total ionization fails to give a true measure of the particle energy.As a high velocity particle passes through the argon gas, \delta E is 780 keV. Fission fragments' passage through the argon gas gives \delta E of 2.5 MeV for light fission fragments(of energy 98 MeV), and 4.2 MeV for heavy ones (of energy 67 MeV). Those measurements are not an absolute evidence concerning the ionization of the fission fragments, but they give an idea for researchers who are working in collecting calorimetric or mass spectrographic data. <ref name="Valentine">J.M. valentine, C.Curran, Reports on progress in physics,21, 1 (1958) 1-29 </ref >
Signal Size
The signal of the detector appears as a result of the electron multiplication, so 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 the electronics used to detect the signal do as well. The figure shows a fast single photon pulse combined with two noise peaks caused by the electronics. <ref name="Chenchik2004"> R.Chechik,A.Breskin,C.Shalem, NIM A535, (2004) 303-308 </ref >
Furthermore, the signal reflects 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 >
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.
Radiation Backgound for Th-232 and U-238 Decay
The background energy spectrum for Th-232 has a main peak for alpha decay of energy 4082.8(14) keV combined with gamma rays of energy 63.81(1) keV and intensity 26.3(13) percent.<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 >.
U-238 has also a close background spectrum as being mainly an alpha emitter, but decay alpha energy is 4274(5) keV with less gamma
intensity 1.02(15) percent when its energy is 113.5(1) keV.<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_d"> http://home.fnal.gov/~hannahnp/decay/decay.html, Jan.6 2011,http://atom.kaeri.re.kr/cgi-bin/decay?Th-232%20A </ref > |
![]() .<ref name="U-238_d"> http://home.fnal.gov/~hannahnp/decay/decay.html, Jan.6 2011,http:http://atom.kaeri.re.kr/cgi-bin/decay?U-238%20A </ref > |
The ionization from alpha particles and gamma rays is considered negligible compared to the one for any fission fragment,because of the enormous difference in mass, charge and kinetic energy. But when the fission fragments are absent, there will always be a signal representing the alpha decay.
Neutron fission X-sect for U-238 and Th-232
The cross section is the proportionality constant for the relationship between the particle travelling 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 gives the value of the cross section for each energy and shows 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 taking place spontaneously or under certain experimental conditions. An incident neutron with a certain kinetic energy hits a nucleus, if the energy is enough to go over Coulomb barrier then the neutron produces new nuclei (fragments) and particles. The new products interact with the surrounding medium depending on their energy and the type of the surrounding medium.
Neutrons are classified depending on their kinetic energy into three types: thermal, intermediate, and fast neutrons. The following table shows the range of each type and additional types of neutrons that are important in applications with 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 name="Dostal">General principles of neutron activation analysis, J. Dostal and C. Elson,p 28 Figure 2.3.</ref> <ref name="James">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 </ref>
U-238 and Th-232 belong to the actinides. They are characterized with relatively high neutron fission cross section for fast neutrons (specifically for neutron energy higher than 1.5 MeV ).
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Efficiency and Sensitive THGEM
The efficiency of a detector is the ratio of the detected events to the total number of events emitted by a source. The intrinsic efficiency has the same definition but it is independent on the detector geometry, and basically considers the interaction cross section of the incident particle with the detector material.<ref name="William"/>
Efficiency requires maximizing the rate of interactions by an appropriate choice for neutron fission sensitive material and the thickness of the sputtered layer over the THGEM cards. Also increasing ionization rate for the fragments under a certain pressure in a highly dense electric field environment, the last condition demands a thorough study for the voltage applied on THGEM cards and distances between them.
Sensitivity is a scale for the detector's ability to output a usable signal as a result of a particle interaction within the detector.<ref name="James"> 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.</ref>
The sensitivity of THGEM as a neutron detector mainly depends on neutron energy range, material cross sections for ionization, neutrons rates of interactions, detector mass, background, noise (sparking and electronic devices used to collect the signal), materials surrounding the detectors and the strength of the electric field.<ref name="James"/> <ref name="William">William R. Leo,Techniques for nuclear and particle physics experiments,1st edition,Springer Verlag, 1995.</ref>
THGEM detector is going to give a digital signal, coming from a defined localized point on the read out plate. This will give information about the real sensitive area of detector to the detected electrons which will be considered a reference for a new size of the future detectors used for the same purpose.
Furthermore, the detected signal will show the energy(s) where the detector is going to be the most sensitive if a calibration curve is available.
Expectations
THGEM as a neutron detector physical properties will be investigated. A clear experimental procedure to get the maximum gain with the least sparking through the detection process. Also, a comparison between the detector efficiency using Th-232 and U-238 and the effect of thickness of coated material on the efficiency. Finally, recommendations will be suggested to improve the next neutron detector design.
For Later
Manufacturing Environment and Detector Aging
Gaseous Detector manufacturing environment is always established to avoid any external effect that may interrupt the detector work. These effects come from the surrounding atmospheric environment of the detector like oxidization and dust accumulation. Also the effects appear in the working environment for the detector as it is surrounded with a noble gas like argon with contaminants under a high electric field effect; this produces chemical radicals attracted to the anode and causes non conducting anode. Finally, coating the THGEM cards by deposition used under a high electric field produces secondary electron emission from the coating, as a result reducing the life time of the of the THGEM cards.<ref name="Grupen">Particle Detectors, Claus Grupen & Boris Shwartz, 2nd edition, 2008 </ref>
Decrease the effect of the detector aging can relatively be controlled. Working in a clean room helps to avoid the contaminants carried by the surrounding atmosphere, and adding filters to the gas source decreases the contaminants carried by the gas.
References
<references/>
Paul Reuss ,Neutron physics,L'editeur EDP Sciences,2008.