The Effect of Different Fill Gases on Bragg Curve Spectrometer Detectors.

Transcription

1 The Effect of Different Fill Gases on Bragg Curve Spectrometer Detectors. A dissertation submitted to the University of Manchester for the degree of Master of Science by Research in the Faculty of Engineering and Physical Sciences Charles Robert Bradbury School of Physics and Astronomy

2 Contents List of figures 4 Abstract 6 Declaration 7 Copyright 8 Acknowledgements 9 1. Introduction Ionisation Chambers Detector properties Design considerations Experimental Setup Gas choice Detector Design Signal Processing Results Gas deterioration Pressure dependence Range Determination Electric field dependence...44

3 CONTENTS Electron combination effects Conclusions. 51 Bibliography 53

4 List of Figures Figure 2.1: Layout of a generic transversal gas-filled ionisation chamber p16. Figure 2.2: SRIM track simulations of 107MeV Pd 106 fragments travelling through a 30µm mylar layer p17. Figure 2.3: Variation of ionisation density as a function of distance travelled by an α particle in air p19. Figure 2.4: Layout of a generic Bragg curve spectroscopy setup p20. Figure 3.1: Schematic diagram of chamber setup used p24. Figure 3.2: The first image shows the field shape that is generated by a planar voltage drop. The second shows the homogenising effect of including field shaping rings p26. Figure 3.3: Circuit diagram of the detector p27. Figure 3.4: Typical preamplifier output as seen on scope p28. Figure 3.5: Observed energy spectrum of 252 Cf in 85mbar freon p30. Figure 3.6: Diagram of signal analysis setup p31. Figure 3.7: All four signal stages shown simultaneously on true timescale p32. Figure 4.1: Variation of signal size with age of gas p34. Figure 4.2: Variation of rise time with age of gas p36. Figure 4.3: Variation of rise time with pressure p38. Figure 4.4: Rise time pressure against reduced electric field p39.

5 LIST OF FIGURES 5 Figure 4.5: SRIM calculated ranges for 99MeV Pd 106 fragments into gas p40. Figure 4.6: Variation of signal amplitude with pressure p41. Figure 4.7: Observed energy spectrum of 252 Cf in 70mbar of isobutane p42. Figure 4.8: SRIM calculated ranges for 99MeV Pd 106 fragments into gas alongside calibrated ranges p43. Figure 4.9: Rise time pressure against reduced electric field p44. Figure 4.10: Variation of signal amplitude with reduced electric field strength p45. Figure 4.11: Table showing reduced field strength required for various signal amplitudes p46. Figure 4.12: Variation of electron drift velocity with reduced electric field strength p46. Figure 4.13: Variation of electron drift velocity with reduced electric field strength plotted alongside data from literature p47. Figure 4.14: Variation of signal amplitude with electron drift time p48. Figure 4.15: Variation of signal amplitude with electron drift time (longer timescale) p49.

6 Abstract A Bragg curve spectrometer was built and run using both isobutane and freon independently as fill gases. Analysis of the signals produced by light fission fragments emitted from a 252 Cf source allowed detailed comparison of the gases characteristics when used in detector chambers. Isobutane was found to provide larger signal amplification, however this appeared to deteriorate with the age of the gas, with an observed loss of 0.82(5)% per hour. Freon provided considerably faster signals however the rise time increased over time, at a rate of 0.97(11)% per hour. Attempts to determine the range of fission fragments did not agree with simulated predictions in the case of either gas. Furthermore the electron drift velocity shows a different trend against reduced electric field strength for the two gases. Signal amplitude losses were observed as the drift time increased as a result of recombination effects. A combination coefficient was calculated for both gases and found to be 0.083(7) µs -1 for isobutane and 0.327(15) µs -1 for freon. From this the mean electron lifetime was found to be 12(1) µs for isobutane and 3.1(1) µs for freon. However below reduced field strengths of V.cm -1.mbar -1 isobutane was found to deviate from the expected exponential decay trend, possibly indicating the presence of a new recombination phenomena.

7 Declaration No portion of the work referred to in the dissertation has been submitted in support of an application for another degree or qualification of this or any other university or other institute of learning. Charles Robert Bradbury.

8 Copyright i. The author of this dissertation (including any appendices and/or schedules to this dissertation) owns any copyright in it (the Copyright ) and s/he has given The University of Manchester the right to use such Copyright for any administrative, promotional, educational and/or teaching purposes. ii. Copies of this dissertation, either in full or in extracts, may be made only in accordance with the regulations of the John Rylands University Library of Manchester. Details of these regulations may be obtained from the Librarian. This page must form part of any such copies made. iii. The ownership of any patents, designs, trade marks and any and all other intellectual property rights except for the Copyright (the Intellectual Property Rights ) and any reproductions of copyright works, for example graphs and tables ( Reproductions ), which may be described in this dissertation, may not be owned by the author and may be owned by third parties. Such Intellectual Property Rights and Reproductions cannot and must not be made available for use without the prior written permission of the owner(s) of the relevant Intellectual Property Rights and/or Reproductions. iv. Further information on the conditions under which disclosure, publication and exploitation of this dissertation, the Copyright and any Intellectual Property Rights and/or Reproductions described in it may take place is available from the Head of School of Physics and Astronomy.

9 Acknowledgements First and foremost I d like to thank the whole nuclear department I ve thoroughly enjoyed being part of the group. No matter what time in the afternoon (or indeed morning) I ve reached my physics saturation point, there has almost always been at least one person equally willing to call it a day and go to the pub. For this I am grateful. Naturally however there are some people who need singling out. I d like to thank Gavin Smith, my supervisor for accepting me onto this course and helping and guiding me throughout the year. I d also like to thank Joe Dare who has been on hand to help me virtually every day throughout the whole year, and has been consistently patient, helpful and insightful. Also Andy MacFarlane who has provided invaluable, friendly help with my experiment and has managed to fix all the things in the lab I ve managed to break. There also many other members of the department, in particular Paul Campbell and Andy Pollitt, who have spent much of the year being bombarded with questions and always somehow finding the time to help. Finally I d like to thank my parents for helping me to fund this course, supporting me on this course and indeed supporting me throughout my entire academic career, despite being absolutely clueless on physics from about GCSE level upwards. Thanks mum and dad, I really appreciate it.

10 1. Introduction Identification and analysis of radiation is an important part of both experimental nuclear physics and commercial radiation usage and in many cases the capabilities and reliability of the detectors employed are of paramount importance. Whilst a Geiger-Muller tube merely counts activity levels, more advanced detectors may be required to record the energy, the type of radiation, the atomic mass and even the atomic number of a particle. Gathering such detailed information requires a carefully considered experimental setup. The type of nuclear detector I have used in this project is a Bragg chamber, a type of ionisation chamber in which irradiated particles pass through a gaseous medium, ionising gas molecules. My work studies the experimental effects of using different gas types in the chamber. As will be discussed this is a complex issue to address theoretically, with many factors to consider. As a result most research of this subject has adopted a semi-empirical approach, my work being no exception. This requires building a suitable working detector, and the design, development and implementation of this detector has formed the bulk of my studies. This has allowed me to run tests with different gases to compare the differences in energy resolution, rise-time and signal amplitude in order to highlight operational advantages and disadvantages of different gases. Specifically I have set up my experiment to compare two gases isobutane and freon. Both gases possess desirable properties for use in an ionisation chamber and as a result have frequently been used in experiments. Yet whilst their individual suitability has been proven, and their superiority over other gases has been shown, there appears to be no direct comparison [OGG + 82], [Bre76], [SWH + 81]. By doing so and analysing the different characteristics of both gases my results would aid

11 SECTION 1: INTRODUCTION 11 myself and others when designing a chamber as well as potentially helping to optimise existing detectors. I have begun this report by outlining the workings of a Bragg chamber and addressing the key considerations faced to someone building one. From this I have detailed the design of my chamber and the experimental method employed. Finally I have presented the data and subsequent results and made conclusions on the properties and suitability of the gases.

12 2. Ionisation chambers Particles entering an ionisation chamber travel through a (usually) gaseous medium, subject to an electric field. The gas molecules are ionised by these particles creating positive ions and electrons which travel to the cathode and anode respectively generating a current. Prior to the 1960s ionisation chambers were widely used in nuclear spectroscopy before becoming somewhat superseded by solid state and scintillation detectors. Keeping the detector fill gas contained means that the particles being studied are invariably required to traverse an entrance window (unless the source itself is actually in the chamber), and despite the use of extremely transparent windows the energy straggling experienced hinders the energy resolution of the chamber [SDH + 74]. Furthermore, collecting the charged particles in a gas is a relatively slow process. This results in lengthy signal rise times, adversely affecting the timing resolution and signal size [Kno99]. For studies of heavy ions however, ionisation chambers remain the most practical type of detector for the following reasons; [i] heavy ions are more capable of causing radiation damage to solid state detectors. However ionisation chambers are not especially subject to radiation damage and if necessary a constant gas flow can be used to keep the gas pure, [ii] physical size is less of a concern; chambers can be built to appropriate dimensions. This gives ionisation detectors more flexibility than other types and allows the construction of larger chambers, with large cross sections. This is especially useful for the study of fission fragments where events may be relatively rare, [iii] gaseous detectors allow for variation in stopping distance by varying the pressure, meaning a single detector can be used for a large range of different particles [SDH + 74], [Kno99]. Gaseous chambers became even more advantageous with the introduction of Bragg Curve Spectroscopy (BCS) by Gruhn et. al. and also Schiessl et. al. in the early eighties [GBL + 82], [SWH + 81]. By setting up the chamber such that the incoming

13 SECTION 2: IONISATION CHAMBERS 13 particles travel parallel to the electric field, the specific ionisation (de/dx) along the particle s track can be observed as a variation in charge collection on the anode. Such an approach allows for improved energy resolution and in theory the determination of the atomic charge number, Z [GBL + 82]. In the following section the general properties of a detector have been outlined and the characteristics which are of greatest significance for a comparison of fill gas types have been discussed. 2.1 Detector Properties. W.R.Leo describes seven key properties of a detector which have varying importance depending on the detector s purpose [Leo94]. They are briefly outlined below Sensitivity The sensitivity of a detector is its ability to detect radiation of a specific nature and energy and is dependent on a number of different factors. A thick entrance window for instance would prevent some particles from entering, resulting in poor sensitivity for low energy or heavily ionising particles. The properties of the fill gas may also affect the sensitivity oxygen for example has a high electron affinity and will soak up electrons as they travel to the anode, reducing the size of the signal. A detector must always be configured such that it offers maximum sensitivity to the radiation it is expected to detect. This allows the observation of large signals, resulting in a good signal-to-noise ratio and improved energy resolution [Leo94] Response A detector creates a charge pulse as charged particles are collected. More energetic particles will cause more ionisation resulting in more charge being collected, however the relationship between energy and pulse size may not be linear. If the

14 chamber is set up such that incoming particles are fully stopped within the detector and all the charge is collected (i.e. limiting effects such as recombination of charged particles and electron attachment) an inherent lack of linearity is avoided. This can be difficult to achieve however, as some recombination and/or attachment is unavoidable. Other effects such as ballistic deficit (the loss of signal from capacitance decay while the charge is still being collected) may also become significant [Leo94] Energy Resolution A beam of monoenergetic particles (such as from a good alpha source) should produce identical signals. However in reality these particles will be measured as having a distribution of energies. This distribution is described by its full width at half maximum (FWHM), which measures the width of the signal at 50% of the maximum amplitude [Wol11]. By minimising this, a detector can provide more reliable information about a particle s energy. When examining fission fragments the energy spectrum should exhibit a characteristic double peak structure, which becomes poorly defined with increased peak overlap if the energy resolution is large [Leo94]. Resolution is often a vital consideration in experiments, particularly when particle identification is an objective, and previous work has shown significant differences when using different gases [OGG + 82]. However, the quality of the spectrum is determined to a degree by the signal processing method employed, making it a difficult and unreliable means of comparison between gases. It is more practical to instead compare errors on raw signal energy measurements Response function Related to the energy resolution, the response function is the shape of the distribution shown by a detector receiving a monoenergetic source. This might be expected to be Gaussian; however unwanted effects may result in distortion of the curve shape. This is generally more of a problem when detecting lighter particles as effects such as bremsstrahlung may become significant [Leo94].

15 SECTION 2: IONISATION CHAMBERS Response time The signal produced by a particle will not be created instantaneously as the charge must be collected. The rate at which the signal forms can be important, particularly if the detector is required for timing measurements or if the signal is to be digitalised. A fast response time will result in a clear, sharp signal, and minimises effects such as recombination and ballistic deficit. The choice of fill gas has been observed to make significant differences [OGG + 82] Efficiency The intrinsic efficiency is the ratio of particles that enter the detector to those which are subsequently detected. Provided the gas pressure is such that all particles are stopped, and the electric field is large enough to collect all the charge, all gases should provide the same high efficiency [Leo94] Dead Time This is a measure of how long a detector is insensitive to further radiation after detecting an event. Like the response time this is inherently linked to the drift speed of the ionised particles and provided the collection time is considerably shorter than the average time between events, this is not an important consideration [Leo94]. An experimental setup allowing assessment of detector sensitivity, energy resolution and response time is therefore required to allow firm conclusions to be drawn about the suitability of each gas for different contexts. By varying the field voltage and gas pressure a more thorough comparison can be made, allowing observations of the ideal conditions and the limitations of the gases.

16 2.2 Design Considerations Prior to the introduction of BCS, an ionisation chamber might typically have had the layout shown in Figure 2.1. Incoming radiation passes through a thin entrance window, required in order to keep the detector gas separate. Before reaching the window the particle should only travel through a high vacuum to ensure all energy loss occurs within the chamber. The region between the cathode and the Frisch grid (explained in section 3.2.5) can be referred to as the active detector region. Figure 2.1: Layout of a generic transversal gas-filled ionisation chamber Entrance Window A similar chamber built by H. Sann et. al. in their 1974 paper A Position Sensitive Ionisation Chamber [SDH + 74] used a 2.5μm Hostaphan (a trade name of polyethylene terephthalate or PET ) window, chosen; because of its stability even after many cycles of pressurizing and depressurizing and despite the fact that it is rather inhomogenous ( d/d ±6%) and much thicker than required by the gas pressure actually used. Indeed the entrance window material must satisfy a particularly demanding set of conditions. As well as featuring stability and homogeneity it must be sufficiently robust and airtight, even when only micrometres thin. As will be shown, even a small change in gas pressure can have a significant effect upon the radiation

17 SECTION 2: IONISATION CHAMBERS 17 stopping distance, particularly when dealing with heavy fission fragments. Unsurprisingly then, an obstructive solid can bring them to an abrupt halt. The thickness of the window is therefore of paramount importance to minimise the energy straggling which occurs (i.e. the loss of energy through interaction with the window), potentially ruining the sensitivity of the detector and broadening the energy resolution. Figure 2.2 shows simulated tracks of a typical fission fragment, in this case a 107 MeV palladium nuclei, travelling through PET. Most are stopped after approximately 20μm and the deviations in direction suggest that significant but unpredictable energy losses occur well before this. The simulation used is the Stopping and Range of Ions in Matter (SRIM) computer program [ZBL85]. Figure 2.2: SRIM track simulations of 107MeV Pd106 fragments travelling through a 30µm PET layer. Approximately 100 tracks are shown. [ZBL85]. Clearly, were the source positioned in the chamber there would be no need for an entrance window and these problems would be avoided. However this would allow no opportunity for secondary detectors such as would be required to digitalise the signal or for a time of flight (TOF) measurement, used to determine the atomic mass, A [Dar09]. It is often desirable to have the source positioned well away from the active detector region to collimate radiation and reduce the event rate, minimising the likelihood of particles being detected simultaneously (this is referred to as pile-up and creates irregular signals). If the region between source and

18 detector is not separated from the active detector region then the entire system must be filled with detector gas instead of being kept under vacuum, resulting in significant energy straggling Ionisation and Charge Collection Having passed the entrance window the radiation travels through the chamber and ionises gas molecules. In doing so it loses energy. The specific ionisation, equivalent to the rate of energy loss of the particle de/dx, varies along the track in accordance with the Bethe-Bloch equation: Equation 2.1: The Bethe-Bloch equation for non-relativistic particles. is the particle s velocity, is its electronic charge, is the mass of an electron, N A is Avogadro s number and A, Z and are the atomic mass number, atomic number and density of the stopping material, respectively. I is the mean energy required to ionise an atom in the medium [Lil01]. Equation 2.1 shows how the specific ionisation depends on the nature of both the particle and the medium it travels through. The velocity dependence means that the specific ionisation also varies with the distance travelled through the medium. The nature of this variation is shown in Figure 2.3. The presence of Z in this equation shows that by fixing all other variables, detailed observation of the particle s behaviour could in theory lead to identification of the particle s atomic number as was discussed in section 2.0. The ionised particles and electrons then drift to the cathode and anode respectively creating a current. If the electric field is perpendicular to the particle trajectories, such as in Figure 2.1, the chamber is referred to as a transversal chamber [OGG + 82]. Note also the presence of a Frisch grid, a fine mesh of wires held at a high positive voltage to generate the electric field. When the electrons reach the grid they then drift to the anode which is held at an even higher voltage. Up to this point however the grid shields the anode from the motion of charge. As a result the

19 SECTION 2: IONISATION CHAMBERS 19 positional dependence of the signal size is removed and all the electrons effectively appear as if they originated from the grid [Lil01]. Figure 2.3: Variation of ionisation density as a function of distance travelled by an α particle in air (the Bragg curve) [Lil01]. It should be observed that provided an incoming particle is fully stopped within the chamber the initial energy of the particle, the total number of ionisation events and the current generated across the electrodes are all (theoretically) proportional. Furthermore since ionisation creates charged pairs, half the current is a result of ion motion and half is due to electron motion. It is therefore only necessary to collect signals on the anode. This is important as the positive ions drift approximately 1000 times slower than the electrons and if collected would greatly hinder the rise time of the signal [Lil01] Bragg Curve Spectroscopy BCS allows more information to be gathered from incident radiation. Instead of a transversal setup, a chamber can be built with axial orientation such that the radiation travels parallel to the field direction. A typical BCS chamber is shown in Figure 2.4.

20 Figure 2.4: Layout of a generic Bragg curve spectroscopy setup [SWH + 81]. The layout allows field shaping rings to encircle the particle trajectories. These are held at uniformly increasing voltages to improve the homogeneity of the electric field [SWH + 81]. Since it is only necessary to collect the electrons, a dedicated cathode is not in fact necessary. The grounded metal wall and entrance window at the front of the chamber serve this purpose. Superimposed upon the image is the Bragg curve of a radiated particle, fully stopped after travelling a distance x through the gas. The electrons produced continue in this direction towards the anode. As the drift speed of the electrons is relatively slow the ionisation events may be treated as occurring simultaneously along the entire trajectory. Consequentially the signal shape will match that of the Bragg curve. Since the shape of the Bragg curve is predominantly dependent on the particle s atomic number [SAD + 93], it can be used to partially identify the radiation. However this is difficult to accurately achieve. Complete particle identification would in addition require mass analysis achieved through other methods, e.g. a TOF setup. This requires a timing detector at a known distance from the ionisation chamber so that velocity measurements can be made. The energy is determined from the ionisation chamber and the mass can then be calculated.

21 SECTION 2: IONISATION CHAMBERS 21 Axial chambers also offer a better energy resolution than transversal chambers due to their symmetry about the fragment trajectories. Transversal chambers will necessarily distort the electric field about the entrance window, and this effect becomes more pronounced with larger windows [OGG + 82].

22 3 Experimental Setup 3.1 Gas Choice Gas properties As mentioned, both isobutane and freon, have both previously been shown to be suitable detector fill gases. This means that they adequately provide the requirements outlined in section 2.1 and do not present any major undesirable effects. Such effects could include corrosivity such as exhibited by isopentane [OGG + 82]. It is difficult to predict how gases will perform in an ionisation chamber because of the complexity of the process they are involved in. The gas stops the incoming radiation (and in doing so forms ionised charge pairs) then provides a medium for the charged particles to drift through the electric field during which time it will absorb some electron through combination reactions. Particularly important properties include the stopping power and ionisation energy of the gas. The stopping power determines the pressure required to stop the radation over a certain distance and this in turn affects the drift speed of the electrons. The ionisation energy affects how much energy is required to create a charged pair, and thus how much charge is collected per fission fragment. A high ionisation energy also increases the likelihood of recombination (i.e. the electron recombining with a positive ion before reaching the anode). The electron affinity of the gas determines the ability of a neutral molecule to collect an additional electron [Kno99].

23 SECTION 3: EXPERIMENTAL SETUP Isobutane Isobutane (also named methylpropane) is an alkane with the chemical formula C 4 H 10. It is therefore an example of an organic compound, specifically a hydrocarbon. Isobutane has a molecular mass of 58 g mol -1 and offers a relatively low ionisation energy of 10.68eV [Nis11a] Freon Freon is the DuPont trade name for chloroflourocarbons [DuP11]. The name is therefore used trivially for a wide range of chemicals but throughout this work it refers specifically to freon 14 (R-14) formally known as tetrafluoromethane or carbon tetrafluoride, and with the chemical formula CF 4. It is much heavier than isobutane with a molecular mass of 88g/mol and also has a higher ionisation energy of 16.2eV [Nis11b]. Chloroflourocarbons are also examples of organic compounds. 3.2 Detector Design Overview The design of the detector used primarily follows features outlined in section 2, most fundamentally the axial orientation. This allows Bragg curve spectroscopy and renders the chamber suitable for further work involving mass measurement although this has not formed part of this experiment. Without a secondary detector signal digitalization is not possible, instead a high frequency oscilloscope has been used for analysis. The detector layout is shown in Figure 3.1.

24 Figure 3.1: Schematic diagram of chamber setup used. Sections have been cut away to show internal detail Source The source used is 252 Cf with a current activity of approximately 100μCi. The age of the source and the half life of californium (approximately 18 months and 32 months respectively), were taken into account when determining this activity level [Kra88]. The long beam tube reduces this high level of activity to a more manageable event rate (i.e. a rate at which signal pile-up rarely occurs). It also collimates the radiated particles to ensure no trajectories pass outside the field shaping rings. Were this to happen the particle would only deposit some of its energy within the detectable area. As is discussed in section 3.3.3, 252 Cf demonstrates a distinctive fission fragment energy spectrum with a twin peaked structure. This structure deteriorates with the age of the californium. An older source was originally used but this provided a poor spectrum with almost complete overlap of the peaks, making it harder to assess the performance of the detector and harder to identify the radiation represented by the spectrum. It would also have caused problems for energy resolution comparisons.

25 SECTION 3: EXPERIMENTAL SETUP Beam Tube and Intermediate Section This region must be completely evacuated to prevent any ionisation taking place before particles reach the window. A rotary pump and turbo pump are used to achieve this. There is a bypass tube which allows these pumps to also evacuate the Bragg chamber. When the turbo pump is activated there is a slight pressure difference between gauges 1 to 2 (with typical readings of 1 x 10-6 mbar and 6 x 10-4 mbar respectively), due to their proximities to the turbo pump. SRIM tests indicate that even a pressure as high as 1 x 10-2 mbar would scarcely affect the fission fragments over the metre or so that they travel. In the set up used, the central section acts as merely an extension to the beam tube. However its inclusion provides the optional inclusion of a secondary detector in further experimentation, either for TOF measurements or as a trigger for signal digitalisation Entrance Window A 0.5μm mylar window was initially used ( mylar is a trade name of PET), however this proved too thin to hold the required pressure gradient. With a chamber pressure above about 50mbar the pressure in the central section was observed to rapidly rise to unacceptably high levels (i.e. one which would result in significant energy straggling). This was therefore replaced with a 0.9μm window which was observed to hold the central section below 2 x 10-6 mbar even with chamber pressures as high as 85mbar. As shown in Figure 3.1 a metal grid is used to support the window at these relatively high pressure gradients. This is kept as thin as possible to maximise the window s transparency. In addition pressure changes are applied at only 1mbar per second to further minimise the stress on the mylar. SRIM simulations predict that a 107 MeV palladium-106 fragment loses approximately 8.11 MeV in the window.

26 3.2.5 The Electric Field and Chamber Casing The electric field is required to collect the electrons. As discussed in section 2 there is no need to collect the positive ions, they are simply allowed to drift to the front of the chamber. As demonstrated by Figure 3.2, field shaping rings are included to homogenise the electric field. Without the rings the electron drift speed would have positional dependence. Figure 3.2: The first image shows the field shape that is generated by a planar voltage drop. The second shows the homogenising effect of including field shaping rings. The Frisch grid acts as the anode while the electrons are being collected. However once the electrons reach the grid it must be ensured that they then proceed to the anode. This is achieved by firstly making the grid as transparent as possible and secondly by making the electric field between the anode and grid (E AG ) larger than the field between the grid and cathode (E GC ). The grid is constructed from 50μm gold-plated tungsten wires with a separation of 2mm. This provides a geometric transparency of 97.5%. As shown by the resistor values in Figure 3.3, there is a E AG /E GC value of The resistor chain in the grid circuit steps down the voltage evenly between the shaping rings.

27 SECTION 3: EXPERIMENTAL SETUP 27 Figure 3.3: Circuit diagram of the detector. Typical HVs ranged from 250V to 1200V. There are 12 rings spaced 1cm apart. There are also 1cm gaps between the window and the first ring, the last ring and the grid and the grid and the anode, giving a total detector length of 14cm. The window to grid distance is therefore 13cm. The internal diameter of the rings is 15cm, which means there is a total active detector volume of 2297cm 3. The total volume of the chamber, including any freely connected piping, is approximately 14800cm 3. This volume is significant because it determines the mass of detector gas subjected to deterioration Pre-Amplifier The pre-amplifier used is an ev-5091 single charge-sensitive hybrid preamplifier made by ev products and modified in the lab to increase the gain from 1mV/pC to 1.5mV/pC. The capacitance fall time is 25µs [End11]. This relatively fast decay time minimises the dead time of the detector and reduces the likelihood of signal pile-up. However it increases the ballistic deficit (the loss of signal from capacitance decay). The extent of this effect is examined in section 4. The pre-amplifier is housed inside the chamber for two reasons. Firstly by minimising its proximity to the anode there is less opportunity for charge to dissipate and/or pick up noise. Furthermore the metal walls of the chamber act as a Faraday cage and help to shield the pre-amplifier from external electromagnetic signals such as radio waves.

28 3.3 Signal Processing Raw signals The output signal from the preamplifier is a few hundred mv high and rises rapidly over a microsecond scale. A typical signal as seen on the oscilloscope is shown in Figure 3.4. Figure 3.4: Typical preamplifier output as seen on scope. Each square represents a height of 200mV and a width of 4µs. The signal is the rapidly rising section of the trace. The tail which follows is simply the capacitor decay of the pre-amplifier. Identifying the rise time of the peak is made more difficult by the smoothing of the signal at the start and end of the pulse (a consequence of component capacitance). A reasonable measurement to make is therefore the time taken for the amplitude to go from 10 to 90 percent of its full height. This is the part of the signal identified in Figure 3.4 by two dotted lines. It was observed in section 2 that the signal should form the mirror image of the fission fragment s Bragg curve as it ionises the detector gas. This is indeed the case, however fission fragments constitute relatively low energy radiation and rather than

29 SECTION 3: EXPERIMENTAL SETUP 29 seeing the characteristic curve shown in Figure 2.3 we instead simply see the final part of the curve where the particle rapidly loses its energy. Studying such a small section of the particle s Bragg curve makes Z identification well beyond the scope of this experiment Signal Amplification In Figure 3.4 the scope has been triggered on a single raw signal, straight from the preamplifier. This is then further processed by a shaping amplifier which differentiates the signal to iron out low frequency noise, and integrates it to negate the effects of high frequency noise. This is important as the signals are subjected to potentially significant voltage variations resulting from mains interference and from radio signals penetrating the chamber casing and being detected by the preamplifier. Selecting an appropriate shaping time ensures the amplification is linear Energy Spectra Amplified signals can then be measured and counted by a data acquisition card connected to a PC. This can be presented graphically as a fission fragment energy spectrum such as shown in Figure 3.5.

30 Figure 3.5: Observed energy spectrum of 252 Cf in 85mbar freon. Tests were run with an anode voltage of 1000V. The low energy spike is predominantly background noise, but may also contain signals created by alpha particles. This shows the nature of 252 Cf fission. Without calibration the channel allocation is arbitrary, but comparison with an equivalent spectrum in the literature [Kno99] reveals that the higher of the two peaks represents nuclei with masses centred about 106 Pd. Fission in the californium source creates pairs of smaller nuclei which are emitted in opposite directions with opposite momentum. Fission events favour emission of a heavy particle and light particle rather than emission of equal fragments [Kra88]. With equal momentum the lighter particles will carry more kinetic energy, resulting in the dual peaked energy distribution shown in Figure 3.5. More specifically it can be shown that if two particles, with masses M 1 and M 2, velocities v 1 and v 2 and kinetic energies E 1 and E 2 (where with equal and opposite momentum (i.e. ), are emitted ), then the kinetic energy of one particle may be given as:. The two peaks therefore correspond to the most commonly emitted light particles and heavy particles respectively. Studying the energy spectrum is a useful way of checking that the detector is working as expected.

31 SECTION 3: EXPERIMENTAL SETUP Signal Identification and Measurement As previously mentioned, without a TOF setup to measure mass or sufficient information about the Bragg curve to identify Z, the Bragg chamber is not capable of identifying fission fragments. However to make comparisons of signal quality across different gases and conditions the same particle must be observed each time. This is achieved through the setup shown in Figure 3.6. Figure 3.6: Diagram of signal analysis setup. The oscilloscope used is a Tekronix TDS 3054C model with a 500 MHz bandwidth and a 5.0GS/s sample rate. The amplifier is an Ortec 571 model, the PSA is an ORTEC 552 model and the gating unit is a Canberra 1454 model. Raw signals such as that shown in Figure 3.4 are amplified. The amplified signals are then sent to both a Pulse Shape Analyser unit and a linear gate unit. If the amplitude of an incoming signal is within a user-defined range then the PSA will emit a pulse which then serves as a coincidence trigger for the linear gate unit. This unit is set to coincidence gating and therefore only transmits input signals if the PSA has deemed it to be within the acceptable range. By observing the developing spectrum on the PC, the PSA can be tuned such that only the light particle peak is transmitted. By

32 also triggering the oscilloscope on this signal, the raw signal can be observed with the knowledge that it is 106 Pd. The stages of this signal identification process are shown as they appear on the scope in Figure 3.7. Figure 3.7: All four signal stages shown simultaneously on true timescale. Signal 1 is the raw signal, signal 2 is the amplified signal, signal 3 is the PSA output signal used as the coincidence gate and signal 4 is the accepted signal used as the scope trigger. Note that signal 4 preserves the peak amplitude of 2. The scope can then be used to measure both the rise time and amplitude of the raw signal.

33

34 4. Results 4.1 Gas deterioration Initial tests sought to analyse the deterioration of the detector gases over time. This was observed through repeated measurements of both amplitude and rise time with a constant electric field and pressure. Figure 4.1: Variation of signal size with age of gas. Tests run with isobutane at 80mbar and an anode voltage of 1000v and freon at 85mbar and an anode voltage of 1000v. Figure 4.1 shows the amplitude of measured signals in relation to the age of the gas, i.e. how long it has been in the chamber and subjected to radiation. With isobutane there is a clear, apparently linear reduction in signal size over the

35 SECTION 4: RESULTS 35 timeframe shown, to the extent that around a fifth of the signal is lost over a 24 hour period. More specifically the signal size is reduced by 0.014(1)% per minute, or by 0.82(5)% per hour. Extrapolating backwards indicates the detector is capable under these conditions of achieving signal sizes of 589(5) mv with completely fresh gas. Experiments which use isobutane for long periods of time generally employ a continuous gas flow system. Typically this might ensure the isobutane is refreshed on average every 3 hours, which from these results would mean a percentage signal loss of approximately 2.47(15)%. These results are comparable to those found by A. Oed et. al. [OGG + 82] who determined a deterioration rate of approximately 1% signal amplitude loss per hour. It must be considered that the observed change in signal size may not be entirely down to gas deterioration. If there exists a small leak in the chamber then air may enter and reduce the purity of the gas. However, close monitoring of the pressure shows no change throughout each experimental run. Furthermore, the observation that freon does not lose signal height suggests that the changes seen are characteristic of the isobutane. In further tests (i.e. tests observing characteristics of the gas other than its deterioration with time) the isobutane used during this experiment has invariably been at different levels of freshness for each measurement. The resulting signal losses have therefore been accounted for by corrections to each measurement, i.e. the corrected amplitude is given by: Equation 4.1 (Where T is the time (in minutes) for which the gas has been in the chamber.) This formula is well supported by its effect on further data sets. When applied to other plots (such as signal size vs. reduced electric field strength), data points taken whilst using older gas show far more agreement with data points taken whilst using fresher gas. However since there is an error associated with this correction results are optimised by keeping T as low as possible i.e. regularly refreshing the gas.

36 Freon demonstrates good stability, showing far less deterioration over time. However the initial amplitude is inferior to that seen with isobutane, suggesting that provided the gases are less than approximately 23 hours old, isobutane offers larger signals and hence a better signal to noise ratio. Furthermore, in Figure 4.2 we see an increase in rise time when using freon, an effect not seen with Isobutane. Figure 4.2: Variation of rise time with age of gas. Tests run with isobutane at 80mbar and an anode voltage of 1000v and freon at 85mbar and an anode voltage of 1000v. The limited data makes this difficult to analyse, but again the observed trend is fitting with other data sets where rise times have been adjusted accordingly. Using corrections based on Figure 4.2 improves the strength of observed correlations in the data. The rise time increase is measured as 0.016(2)% per minute or 0.97(11)% per hour. The correction is therefore: Equation 4.2 The size of the associated error makes it important to keep the freon reasonably fresh. The 106μCi activity rate of the source equates to approximately 7800 particles reaching the detector every second. Approximately 97% of these will be alpha

37 SECTION 4: RESULTS 37 particles which, with the chamber at the pressures used, will be stopped in the anode rather than the gas [Kno99]. However, even assuming that all the particles deposit all their energy in the gas they would only ionise something in the region of molecules over a 24 hour period less than % of the gas. It would be naïve then to assume that the changes to the signals are consequences of radiation damage. It is likely that the deterioration is in fact due to contamination of the gas from the chamber itself, either from dust or material out-gassing. Even in very small quantities this may significantly affect the gas properties, depending on how sensitive the ionisation, recombination and mobility mechanisms are to impurities. To gain further insight into this phenomenon a wider range of gases might be studied to determine how this deterioration depends upon properties such as ionisation potential, electron affinity and molecular mass. Repeating the experiment in a different chamber with different components could show the extent to which the changes are dependent on the gas surroundings. 4.2 Pressure dependence Changing the pressure in the chamber has two effects on fragment detection. Firstly the stopping power of the gas is increased, reducing the range of the particles. Secondly the drift speed of the electrons is reduced. These changes have opposite effects upon the rise time a reduced range shortens the window over which electrons must be collected reducing the rise time, whilst a slower drift speed lengthens the rise time. Figure 4.3 shows that the two gases have varying sensitivity to these effects, with the rise time increasing with pressure in the case of isobutane and decreasing with pressure in the case of freon.

38 Figure 4.3: Variation of rise time with pressure. Tests run with an anode voltage of 1000V. To observe the effect of pressure changes on just the drift speed, the range variation must be accounted for. To achieve this, a linear relationship between pressure and range is assumed and the product of rise time and pressure is now plotted. This can be plotted against the reduced electric field (E/P) to generalise the results as shown in Figure 4.4.

39 SECTION 4: RESULTS 39 Figure 4.4: Rise time pressure against reduced electric field. Tests run with an anode voltage of 1000V. We see that isobutane demonstrates a greater sensitivity to the reduced field size over this range of conditions (i.e mbar at 66Vcm -1 ). It is likely however that over a wider range the trend becomes non-linear and the gradients in Figure 4.4 are only representative of the gases under these reduced fields. 4.3 Range determination To calculate the drift velocity of the electrons we must first determine the range of the particles in the gas. With the setup used there is no way of directly identifying this, and as such the range values rely on [SRIM] calculations. However it is possible to determine the minimum pressure, (the pressure below which the range exceeds the chamber length). It should be considered that whilst the full chamber length is 13cm there is an additional 1cm gap between the Frisch grid and the anode. This region should be avoided as electrons ionised here will create less charge on the anode than those which have drifted the full 1cm gap. However, since this would

40 only affect the tail of the signal where the ionisation density is lowest, noticeable changes to the signal are not seen until the pressure is low enough for particles to reach the anode itself. Range simulations using SRIM predict the pressure dependences shown in Figure 4.5. Figure 4.5: SRIM calculated ranges for 99 MeV Pd 106 fragments into gas. Note that the fragment energy has been reduced to account for energy straggling. The fragments have an initial energy of 107 MeV, but lose approximately 8 MeV in the entrance window. This is determined by [SRIM] calculations. From this we should expect significant changes to the signals to occur at pressures of 52mbar and 66mbar for isobutane and freon respectively, with Frisch grid related deviations potentially occurring at 56mbar and 71mbar. Figure 4.6 shows the changes seen to signal size.

41 SECTION 4: RESULTS 41 Figure 4.6: Variation of signal amplitude with pressure. Tests run with an anode voltage of 1000V. In both cases we see the signal drop at pressures well above that predicted by the SRIM simulations below approximately 78mbar for isobutane (assuming the peak of this data set to be approximately midway between the two data points with the largest signal amplitudes), and 82mbar for freon, suggesting the fragments are exhibiting a longer range than predicted. There are two pieces of evidence to support this finding. Firstly, studying the energy spectra of tests conducted below these pressures shows a shift in percentage yield from light to heavy particles. An example is shown in Figure 4.7.

42 Figure 4.7: The solid line shows the observed energy spectrum of 252 Cf in 70mbar of isobutane. Tests run with an anode voltage of 1000V. The dotted line is taken from Figure 3.5 and shows the observed energy spectrum of 252 Cf in 85mbar of freon. Compared to Figure 3.5, Figure 4.7 shows a higher ratio of heavy to light particles, strongly suggesting that many of the lighter particles (with greater range than the heavy particles) are not depositing all their energy before reaching the anode. Secondly, at these lower pressures there is an observed lack of amplitude dependence on signal rise times. Although the electronics setup is designed to restrict the signals to just that of the energy corresponding to the light particle peak, there is still a small tolerance (this variation constitutes the most significant contribution to the experimental energy resolution.) Plotting the individual signal rise times against their amplitudes shows a positive correlation, with more energetic particles creating a longer track and hence the detector taking more time to collect the ionised charge. However at pressures below 78mbar and 82mbar for isobutane and freon respectively this trend ceases to exist and the signal rise times appear constant and independent of the amplitude. This indicates that more energetic

43 SECTION 4: RESULTS 43 particles are ionising the same track-length as less energetic particles, which would be the case if they are were reaching the anode. Assuming then that this data is correct, the predicted ranges are 67% (in the case of isobutane) and 80% (in the case of freon ) of that observed. This discrepancy may be due to problems with the SRIM model under the conditions used in this experiment. By using these as reference points the SRIM predictions can be calibrated allowing range estimates to be made. Figure 4.8 shows the predicted SRIM ranges alongside the same data calibrated to my results. Figure 4.8: SRIM calculated ranges for 99MeV Pd 106 fragments into gas alongside calibrated ranges. The pressures actually used for observing the electric field dependence and drift speed patterns were 80mbar for isobutane and 85mbar for freon. From Figure 4.8 the actual range of the particles can be estimated to be approximately 13.5cm in isobutane and 13.6cm in freon at these pressures. This means that in both cases some electrons will actually be ionised behind the Frisch grid, but the charge loss will be small and will only affect the electrons ionised at the very end of the