H Proposal Defense

• Introduction
• Gas Electron Multiplication
• Detector Signal Size
• Summary

Topic

The Performance of Thick Gaseous Electron Multiplier Preamplifiers (THGEM) as a Neutron Sensitive Detector.

Introduction

I propose to construct and measure the performance of a fission chamber instrumented with preamplifiers known a Thick Gas Electron Multipliers (THGEM). This fission chamber is a chamber filled with an inert gas 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 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 enter the THGEM preamplifier producing secondary electrons and directed to charge collectors using strong electric fields.

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 microns of copper. The copper clad kapton is perforated with 50-100 micron diameter holes separated by 100-200 microns in a staggered array . The THGEM preamplifier is more macroscopic version of the GEM foil that uses 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 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.

Gas Electron Multiplication

An electron from a primary ionization event needs to liberate more charge in order to create a detectable signal. This amplification is accomplished using strong electric fields to accelerate the primary electron giving it sufficient energy to liberate more electrons from the chamber gas. In a traditional drift chamber, an electric field is established between two or more conducting wires. Asan electron is accelerated towards a wire, it may obtain sufficient energy to ionize the chamber gas producing secondary freed electrons. The secondary electrons produced may themselves also be accelerated along the electric field lines and cause further ionization in a process commonly referred to as an avalanche.

The Gas Electron Multiplier (GEM)

The Gas Electron Multiplier (GEM), invented by Fabio Sauli<ref name="Sauli1997"/> in 1997, is a modern method of using electric fields to create an avalanche condition. A GEM is 50 microns thick kapton foil clad on both sides with 5 microns of copper. A staggered pattern of 50 microns diameter holes, equally spaced by distances comparable to the hole diameter, is etched into the copper clad foil. The small size facilitates the use of low voltages (300 Volts) to generate an 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.

Fig.1 GasElectron Multiplication

Fig.2 Electric filed distribution in the holes<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 >

Using a GEM has advantages and disadvantages. The micrometer scale of a GEM foil makes it susceptible to damage from sparking at high amplification voltages. When the voltage increases from 525V to 600V regardless of the pattern geometry,the 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.

Fig.3 Different types of GEM <ref name="A.Bressan1999"/>

Fig.4 Gain and sparking as function of Voltage<ref name="A.Bressan1999"/>

Fig.5 Gain for canonical GEM<ref name="A.Bressan1999"/>

Fig.6 Gain for CAT GEM<ref name="A.Bressan1999"/>

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.

The Thick Gas Electron Multiplier (THGEM)

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="Agocs"/> Each card is chemically etched to leave a copper trace around the perimeter of the card as a border for a square area of a side length of 10 cm as shown in fig. 7.

Fig.7 THGEM preamplifier before applying the paste

A thin layer (5 microns) of resistive paste (ED-7100) is applied to the card to permit current from flowing on the surface. Then the card is machined to have 0.5 mm diameter holes. 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 at 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 fig.8

Fig.8 THGEM's chamber

The two plastic parts are collected together by a number of M3 plastic screws located around the detector window to form a well closed 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 transfers the digital signal to a break board, then to the DAQ system.

Detector Signal

Ionization

Ionization is the liberation of electrons 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 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="William"/>, regardless of the incident particle type or energy as shown in the following table for 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 Ionization

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 a 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, the researchers tried to measure [math]\Delta E [/math] (ionization defect), and the degree to which the total ionization fails to give a true measure of particle energy using the following equation:<ref name="Valentine"/>

[math] \Delta E = E - I w_{\alpha} [/math]

where E is the energy of the primary particle, I is the amount of ionization it produces, and [math] w_{\alpha} [/math] is the mount of energy spent per ion pair.

For instance, for very high speed argon gas passing through argon gas, [math] \Delta E [/math] is 780 keV. For fission fragments' passage through the argon gas gives [math]\Delta E [/math] of 2.5 MeV for light fission fragments(of energy 98 MeV), and 4.2 MeV for heavy ones (of energy 67 MeV). These measurements are not absolute evidence concerning the ionization of the fission fragments, but they give an idea for researchers who are working on 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 >

The ionized nuclei that are ejected from a heavy nucleus which has undergone the fission process can ionize atoms in the vicinity of the fission event. These ejected nuclei are referred to as fission fragments. The fission fragments have path lengths in matter which are on the order of microns. The energy loss process that these fission fragments undergo over the short range may be astochastically described in terms of three steps; as the fission fragment is totally ionized, fission fragment is exchanging the charge with the gas atoms, and as the fission fragment is totally neutralized.

First, as the fission fragment is totally ionized (without any bounded electrons), the total energy of is represented by :

[math] E = \frac{k Z_1 Z_2 e^2}{r} + \frac{1}{2} M v^2 [/math]

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 fragments, as well as the attraction force between the fission fragment and electrons. But at the moment the first term is dominant,since the fission fragment collision causes the electrons to scatter away from the fission fragment.

Second, when the fission fragment decelerates, a charge exchange starts between the fission fragment and gas atoms, as the electrons are either scattering away from the fission fragment nucleus or attached to the nucleus as it is moving. The total energy loss of the fission fragment suggested by Bohr in 1940 is <ref name="Hyde">Earl K. Hyde,The Nuclear Properties of the Heavy Elements 3,Denver Publications, 1971, p 195-210 </ref> :

[math] \frac{1}{N} \frac{dE}{dx} = \frac {4 \pi e^4}{mv^2} (Z_1^{eff})^2 Z_2 log\frac{1.123mv^3}{w e^2 Z_1^{eff}} + \frac {4 \pi e^4}{M_2 v^2} Z_1^2 Z_2^2 \times log\left(\frac{M_1 M_2}{M_1 +M_2} \frac {v^2 a_{12}^{scr}}{Z_1 Z_2 e^2}\right) [/math]

where N is the number of particles of the stopping medium per cubic centimeter,M1 and M2 are masses of the fragment and the absorber,Z1 and Z2 are the atomic number of the fragment and the absorber, e is the electron charge, v is the fragment velocity, Z1 effective is the charge of the fragment, changes from 20 for the beginning of the track to a value of 2 close to the end of the track. [math]a_{12}^{scr} [/math] is an impact parameter which tells at what distance the energy loss in the nuclear collisions is effectively zero owing to the screening of the charges of the nuclei by the atomic electrons, [math] w = I/\hbar [/math] is the average oscillation frequency of the electrons in the atom.

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 the value of Z-effective at each stage is; 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.<ref name="Hyde"/>

Recently, Bethe's theory is used to estimate the energy loss for fission fragments which involves using diverse model for the effective charge obtained based on experimental data analysis for energy loss.<ref name="Rykov">V.A Rykov, atomic energy,vol83,No.1,1997 </ref>

[math] -\frac{dE}{dx} [\frac{MeV}{mg/cm^3}]= 3.072\times 10^{-4}\left(\frac {Z^{eff}}{\nu/c}\right)^2 \left(\frac {Z_m}{A_m}\right) ln\left(\frac {m_e \nu^2}{I}\right)[/math]

[math] Z_{eff}=Z \left[1-A exp\left(-B\frac{\nu}{\nu_o Z^{\frac{3}{2}}}\right)\right] [/math]

where Z is the nuclear charge of the fission fragment, [math]'''\nu_o'''[/math] is the speed of the electron in first Bohr orbit, [math]'''\nu'''[/math] is the speed of the ion, [math]Z_m , A_m [/math] are the nuclear charge and the atomic mass of the medium, [math]'''m_e''' [/math] is the mass of the electron, [math]I= KZ_m [/math] is the mean excitation energy of the atomic electrons of the medium of atomic number [math]Z_m[/math]. A,B are the fitting parameters dependent on the medium, for example A=0.92, B= 0.72 for Light fission fragments. A=0.99, B= 0.82 for heavy fission fragments are they are passing Ar/CH4 (95/5 percent) medium.<ref name="Biwas">D. C. Biswasa, M. N. Raob and R. K. Choudhury,NIM,Volume 53, Issue 3, 1 March 1991, Pages 251-254 </ref>

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.

Neutron fission Cross Section 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 value and shows the resonance peaks. Theoretically, there is not any model that gives a detailed prediction of neutron fission 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.<ref name="Hyde"/>

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, and 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
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>

Both of 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 ).

Fig.9 U 238 fission cross section	Fig.10 Th 232 fission cross section

Signal Size

An electron, freed by ionization and immersed in an external electric field, can ionize other atoms in a gas volume as a result of its acceleration in the external electric field. The number of additional freed electrons can be determined in terms of the external electric field (E), the pressure of the surrounding gas (P), and two fit parameters (A & B) which depend on the gas properties as shown in the equation <ref name="William"/>

[math] \alpha= APe^{(\frac{-BP}{E})} [/math]

The parameter [math]\alpha[/math] is known as the Townsend coefficient. In the case of a THGEM preamplifier, an electric field (E) of [math]2 \times 10^4 \frac{\mbox{V}}{\mbox{cm}}[/math] is established when a potential difference of 2 kV is applied between the top and bottom sides of the plate that are 0.1 cm apart. In this work, the gas chamber contains a 1 Atm mixture that is 90% Argon and 10 % C02 by volume. The parameters from reference <ref name="Sharma"> A.Sharma,F. Sauli, first tawsend coefficients measurements for argon gas european organization for nuclear research (1993) </ref > are not given for this mixture. To estimate the THGEM pre-amplification, the A and B parameters for a 96/4 percent mixture gas mixture were used which are 5.04 [math]\frac{1}{\mbox{cm Torr}}[/math] and 90.82 [math]\frac{\mbox{V}}{\mbox{cm Torr}}[/math] respectively .

Using the above equation we would expect [math] (\alpha)[/math] to be about [math]121[/math] per cm (which is a factor of 30 less than the coefficient for a GEM foil). A free electron traveling through the 0.1 cm THGEM hole should produce 12 additional freed electrons due to the Electric field of the THGEM pre-amplifier. We would expect a single freed electron to produce a maximum of [math]10^3[/math] electrons if it traverses three THGEM pre-amplifiers.

The number electrons emitted from a fission fragment is [math]5.1X10^4/cm^3.s [/math] as result of a 30 MeV proton projected toward a U-238 target<ref name=""> M. Facina "A gas catcher for the selective production of radioactive beams through laser ionization" mater thesis, Leuven 2004 </ref > which meets with GEANT4 simulation of Cd-123 ionization when it is traveling in Ar/CO2 gas.(Cd-123 with 81.6 MeV is one of the fission fragment of a fission event for a 1 MeV neutron projected toward U-238).

So the expected signal for the fission fragment detected by 50 ohm oscilloscope is roughly 2-20 mV at 50 ns.

For a single ionization event, the detector signal would be small, while for a fission event the number of freed electrons will be substantially larger resulting in a large signal.

The Efficiency of THGEM

The efficiency of a detector is the ratio of the detected events to the total number of events entering the detector. In order to detect a neutron entering the gas chamber the neutron must induce fission and the resulting fragments must ionize the gas. The cross-section for U-238(n,f) is given above in fig.9. The probability of a fission event for a 1 cm^3 U-238 target varies from 1 to 5 % as shown in fig.11.

Fig.11 The probability a fission event in U 238

The probability of a fission fragment escaping into the ionization gas is also required to calculate the detector's efficiency. The number of fission fragments that escape from the surface of a UO3 film relative to the number of fission events is given by the following formula<ref name="Hudler">J.C.Hudler rdiation measurements 43,S334-S336, 2008 </ref>:

[math] \frac{N}{A} = \frac {\ln(\frac{d}{R_T}+ 0.5)}{8\pi[1-\frac{d}{R_T}] + \frac{d}{R_T}} [/math]

where [math]N[/math] is the number of fission fragments that escaped from the surface per unit area, A represents the number of fission interactions per unit volume.

d: The thickness of film (um).

[math]R_T[/math]: The mean range of the fission fragments (12.07 um) Assuming that the number of fission fragments is :

[math] N = \int_{V_1} A \frac{cos\theta}{4\pi r^2} dV + \int_{V_2} A \frac{cos\theta}{4\pi r^2} dV [/math]

and the average range of the fission fragment through the emulsion [math]R_E[/math] is :

[math] \lt R_E\gt =\lt R_T\gt - \lt r\gt [/math]

where <r> is the fission fragment's range in the thin film.

Fig.12 The probability of a fission fragment escaping into the ionization gas

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 sensitive area of the detector to the detected electrons which will be considered a reference for future detectors used for the same purpose.

Summary

Thick gas electron multiplier (THGEM) is expected to be used as a detector for the fast neutrons and their locations. The detector will be built based on the properties of a neutron fissionable material that has a neutron fission cross section between 0.1 - 1 barn. The fission event ejects stochastically one fission fragment with energy of 65-100 MeV that will lose energy in the form of ionization in a Ar/CO2 gaseous medium that surrounds the fission interaction to free on average [math] 10^4 [/math] electrons. The electrons are collected by a drift electric field through the THGEM preamplifier that will multiply the electrons in three stages to end up having a number of electrons of the order of [math] 10^6 -10^7 [/math] electrons collected by a wired net read out plate to produce a signal of magnitude of 2 - 20 mV.

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.

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"/>

Least fission fragment K.E needed for ionization

Ionization and Number of the the electrons produced

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>

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.

File:Hadler on energy absorption effects in U Th thinfilms.pdf

Sensitivity of the THGEM as neutron detector for gamma radiation is minimized by properties mentioned previously about THGEM low amplification factor, also U-238 as a high cross section for gamma absorption in the desired energy range.

References

<references/>

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