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Gamma and X-Ray Detection Phone contact information Benelux/Denmark (32) 2 481 85 30 • Canada 905-660-5373 • Central Europe +43 (0)2230 37000 • France (33) 1 39 48 52 00 • Germany (49) 6142 73820 • Japan 81-3-5844-2681 • Russia (7-495) 429-6577 • United Kingdom (44) 1235 838333 • United States (1) 203-238-2351 For other international representative offices, visit our web site: http://www.canberra.com or contact the CANBERRA U.S.A. office. 6/06 Printed in U.S.A. Figure 1.1 Efficiency Calibration DETECTOR OVERVIEW The kinds of detectors commonly used can be categorized as: a. Gas-filled Detectors b. Scintillation Detectors c. Semiconductor Detectors The choice of a particular detector type for an application depends upon the X-ray or gamma energy range of interest and the application’s resolution and efficiency requirements. Additional consider- ations include count rate performance, the suitability of the detector for timing experiments, and of course, price. DETECTOR EFFICIENCY The efficiency of a detector is a measure of how many pulses occur for a given number of gamma rays. Various kinds of efficiency defini- tions are in common use for gamma ray detectors: a. Absolute Efficiency: The ratio of the number of counts produced by the detector to the number of gamma rays emitted by the source (in all directions). b. Intrinsic Efficiency: The ratio of the number of pulses produced by the detector to the number of gamma rays striking the detector. c. Relative Efficiency: Efficiency of one detector relative to another; commonly that of a germanium detector relative to a 3 in. diameter by 3 in. long NaI crystal, each at 25 cm from a point source, and specified at 1.33 MeV only. d. Full-Energy Peak (or Photopeak) Efficiency: The efficiency for producing full-energy peak pulses only, rather than a pulse of any size for the gamma ray. Clearly, to be useful, the detector must be capable of absorbing a large fraction of the gamma ray energy. This is accomplished by using a detector of suitable size, or by choosing a detector material of suitable high Z. An example of a full-energy peak efficiency curve for a germanium detector is shown in Figure 1.1. DETECTOR RESOLUTION Resolution is a measure of the width (full width half max) of a single energy peak at a specific energy, either expressed in absolute keV (as with Germanium Detectors), or as a percentage of the energy at that point (Sodium Iodide Detectors). Better (lower FWHM value) resolution enables the system to more clearly sep- arate the peaks within a spectrum. Figure 1.2 shows two spectra collected from the same source, one using a sodium iodide (NaI(TI)) detector and one using germanium (HPGe). Even though this is a rather simple spectrum, the peaks presented by the sodium iodide detector are overlapping to some degree, while those from the germanium detector are clearly separated. In a complex spectrum, with peaks numbering in the hundreds, the use of a germanium detector becomes mandatory for analysis. GAS-FILLED DETECTORS A gas-filled detector is basically a metal chamber filled with gas and containing a positively biased anode wire. A photon passing through the gas produces free electrons and positive ions. The electrons are attracted to the anode, producing an electric pulse. At low anode voltages, the electrons may recombine with the ions. Recombination may also occur for a high density of ions. At a sufficiently high voltage nearly all electrons are collected, and the detector is known as an ionization chamber. At higher voltages the electrons are accelerated toward the anode at energies high enough to ionize other atoms, thus creating a larger number of electrons. This detector is known as a proportional counter. At higher voltages the electron multiplication is even greater, and the number of electrons collected is independent of the initial ionization. This detector is the Geiger-Mueller counter, in which the large output pulse is the same for all photons. At still higher voltages continuous discharge occurs. The different voltage regions are indicated schematically in Figure 1.3. The actual voltages can vary widely from one detector to the next, depending upon the detector geometry and the gas type and pressure. IONIZATION CHAMBER The very low signal output for the ionization chamber makes this detector difficult to use for detecting individual gamma rays. It finds use in high radiation fluxes in which the total current produced can be very large. Many radiation monitoring instruments use ionization chambers. Absolute ionization measurements can be made, using an electrometer for recording the output. 1 PROPORTIONAL COUNTER Proportional counters are frequently used for X-ray measurements where moderate energy resolution is required. A spectrum of 57 Co is shown in Figure 1.5 in which 14.4 keV gamma rays are well-separated from the 6.4 keV X rays from iron. Proportional counters can be purchased in different sizes and shapes, ranging from cylindrical with end or side windows to “pan- cake” flat cylinders. They may be sealed detectors or operate with gas flow, and may have thin beryllium windows or be windowless. A detector is typically specified in terms of its physical size, effective window size and gas path length, operating voltage range and resolution for the 5.9 keV X ray from a 55 Fe source (Mn X ray). Typical resolutions are about 16 to 20% full-width at half maximum (FWHM). Operating voltages depend upon the fill gas as well as the geometry. For X rays, noble gases are often used, with xenon, krypton, neon and argon common choices. Xenon and krypton are selected for higher energy X rays or to get higher efficiencies, while neon is selected when it is desired to detect low energy X rays in the presence of unwanted higher energy X rays. Sometimes gas mix- tures are used, such as P-10 gas, which is a mixture of 90% argon and 10% methane. Gas pressures are typically one atmosphere. The 2006 preamplifier available for proportional counters is shown in Figure 1.4. GEIGER-MUELLER COUNTER The Geiger-Mueller counter produces a large voltage pulse that is easily counted without further amplification. No energy measurements are possible since the output pulse height is independent of initial ionization. Geiger-Mueller counters are available in a wide variety of sizes, generally with a thin mica window. The operating voltage is in the plateau region (see Figure 1.3), which can be rela- Figure 1.2 Figure 1.3 Gas Detector Output vs. Anode Voltage tively flat over a range of bias voltage. The plateau is determined by measuring the counting rate as a function of the anode voltage. The discharge produced by an ionization must be quenched in order for the detector to be returned to a neutral ionization state for the next pulse. This is accomplished by using a fill gas that contains a small amount of halogen in addition to a noble gas. The voltage drop across a large resistor between the anode and bias supply will also serve to quench the discharge since the operating voltage will be reduced below the plateau. The Geiger-Mueller counter is inactive or “dead” after each pulse until the quenching is complete. This dead time can be hundreds of microseconds long, which limits the counter to low count rate applications. Figure 1.4 Proportional Counter and Preamplifier SCINTILLATION DETECTORS A gamma ray interacting with a scintillator produces a pulse of light, which is converted to an electric pulse by a photomultiplier tube. The photomultiplier consists of a photocathode, a focusing electrode and 10 or more dynodes that multiply the number of electrons striking them several times each. The anode and dynodes are biased by a chain of resistors typically located in a plug-on tube base assembly. Complete assemblies including scintillator and photomultiplier tube are commercially available from CANBERRA. The properties of scintillation material required for good detectors are transparency, availability in large size, and large light output proportional to gamma ray energy. Relatively few materials have good properties for detectors. Thallium activated NaI and CsI crystals are commonly used, as well as a wide variety of plastics. LaBr 3 (Ce) crystals are a newer type of scintillation detector material of- fering better resolution, but otherwise, similar characteristics to Figure 1.5 57 Co Spectrum from Counter SEMICONDUCTOR DETECTORS A semiconductor is a material that can act as an insulator or as a conductor. In electronics the term “solid state” is often used inter- changeably with semiconductor, but in the detector field the term can obviously be applied to solid scintillators. Therefore, semiconductor is the preferred term for those detectors which are fabricated from either elemental or compound single crystal materials having a band gap in the range of approximately 1 to 5 eV. The group IV elements silicon and germanium are by far the most widely-used semiconductors, although some compound semiconductor materials are finding use in special applications as development work on them continues. Table 1.1 shows some of the key characteristics of various semiconductors as detector materials: Table 1.1 Element vs. Band Gap Material Z Band Gap (eV) Energy/e-h pair (eV) Si Ge CdTe HgI 2 GaAs 14 32 48-52 80-53 31-33 1.12 0.74 1.47 2.13 1.43 3.61 2.98 4.43 6.5 5.2 Semiconductor detectors have a p-i-n diode structure in which the intrinsic (i) region is created by depletion of charge carriers when a reverse bias is applied across the diode. When photons interact within the depletion region, charge carriers (holes and electrons) are freed and are swept to their respective collecting electrode by the electric field. The resultant charge is integrated by a charge sensitive preamplifier and converted to a voltage pulse with an ampli- tude proportional to the original photon energy. Since the depletion depth is inversely proportional to net electrical impurity concentration, and since counting efficiency is also depen- dent on the purity of the material, large volumes of very pure material are needed to ensure high counting efficiency for high energy photons. NaI detector crystals. NaI is still the dominant material for gamma detection because it provides good gamma ray resolution and is economical. However, plastics have much faster pulse light decay and find use in timing applications, even though they often offer little or no energy resolution. NaI(Tl) SCINTILLATION DETECTORS The high Z of iodine in NaI gives good efficiency for gamma ray detection. A small amount of Tl is added in order to activate the crystal, so that the designation is usually NaI(Tl) for the crystal. The best resolution achievable ranges from 7.5%-8.5% for the 662 keV gamma ray from 137 Cs for 3 in. diameter by 3 in. long crystal, and is slightly worse for smaller and larger sizes. Figure 1.7 shows, respectively, the absorption efficiencies of various thicknesses of NaI crystals and the transmission coefficient through the most commonly used entrance windows. Many configurations of NaI detectors are commercially available, ranging from crystals for X-ray measurements in which the detector is relatively thin (to optimize resolution at the expense of efficiency at higher energies), to large crystals with multiple phototubes. Crystals built with a well to allow nearly spherical 4π geometry counting of weak samples are also a widely-used configuration. A typical preamplifier and amplifier combination is shown in Figure 1.6. Figure 1.6 NaI(Tl) Detector Electronics The light decay time constant in NaI is about 0.25 microseconds, and typical charge sensitive preamplifiers translate this into an output pulse rise time of about 0.5 microseconds. For this reason, NaI detectors are not as well-suited as plastic detectors for fast coincidence measurements, where very short resolving times are required. LaBr 3 (Ce) detectors have a light decay time constant of 0.03 microseconds making them another possible solution for coincidence measurements. Prior to the mid-1970’s the required purity levels of Si and Ge could be achieved only by counter-doping p-type crystals with the n-type impurity, lithium, in a process known as lithium-ion drifting. Although this process is still widely used in the production of Si(Li) X-ray detectors, it is no longer required for germanium detectors since sufficiently pure crystals have been available since 1976. The band gap figures in Table 1.1 signify the temperature sensitivity of the materials and the practical ways in which these materials can be used as detectors. Just as Ge transistors have much lower maximum operating temperatures than Si devices, so do Ge detectors. As a practical matter both Ge and Si photon detectors must be cooled in order to reduce the thermal charge carrier generation (noise) to an acceptable level. This requirement is quite aside from the lithium precipitation problem which made the old Ge(Li), and to some degree Si(Li) detectors, perishable at room temperature. The most common medium for detector cooling is liquid nitrogen, however, recent advances in electrical cooling systems have made electrically refrigerated cryostats a viable alternative for many detector applications. In liquid nitrogen (LN 2 ) cooled detectors, the detector element (and in some cases preamplifier components), are housed in a clean vacuum chamber which is attached to or inserted in a LN 2 Dewar. The detector is in thermal contact with the liquid nitrogen which cools it to around 77 °K or –200 °C. At these temperatures, reverse leakage currents are in the range of 10 -9 to 10 -12 amperes. Figure 1.7 In electrically refrigerated detectors, both closed-cycle mixed re- frigerant and helium refrigeration systems have been developed to eliminate the need for liquid nitrogen. Besides the obvious advantage of being able to operate where liquid nitrogen is unavailable or supply is uncertain, refrigerated detectors are ideal for applications requiring long-term unattended operation, or applications such as undersea operation, where it is impractical to vent LN 2 gas from a conventional cryostat to its surroundings. A cross-sectional view of a typical liquid nitrogen cryostat is shown in Figure 1.8. DETECTOR STRUCTURE The first semiconductor photon detectors had a simple planar structure similar to their predecessor, the Silicon Surface Barrier (SSB) detector. Soon the grooved planar Si(Li) detector evolved from attempts to reduce leakage currents and thus improve resolution. The coaxial Ge(Li) detector was developed in order to increase overall detector volume, and thus detection efficiency, while keep- ing depletion (drift) depths reasonable and minimizing capacitance. Other variations on these structures have come, and some have gone away, but there are several currently in use. These are illustrated in Figure 1.9 with their salient features and approximate energy ranges. For more information on specific detector types refer to the Detector Product Section of this catalog. ©2006 Canberra Industries, Inc. All rights reserved. DETECTOR PERFORMANCE Semiconductor detectors provide greatly improved energy resolution over other types of radiation detectors for many reasons. Fun- damentally, the resolution advantage can be attributed to the small amount of energy required to produce a charge carrier and the con- sequent large “output signal” relative to other detector types for the same incident photon energy. At 3 eV/e-h pair (see Table 1.1) the number of charge carriers produced in Ge is about one and two or- ders of magnitude higher than in gas and scintillation detectors respectively. The charge multiplication that takes place in proportional counters and in the electron multipliers associated with scintillation detectors, resulting in large output signals, does nothing to improve the fundamental statistics of charge production. The resultant energy reduction in keV (FWHM) vs. energy for various detector types is illustrated in Table 1.2. Table 1.2 Energy Resolution (keV FWHM) vs. Detector Type Energy (keV) 5.9 1.22 1.332 Proportional Counter X-ray NaI(Tl) 3 x 3 NaI(Tl) Si(Li) Low Energy Ge Coaxial Ge 1.2 3.0 — 0.16 0.14 — — 12.0 12.0 — 0.5 0.8 — — 60 — — 1.8 At low energies, detector efficiency is a function of cross-sectional area and window thickness while at high energies total active detector volume more or less determines counting efficiency. Detectors having thin contacts, e.g. Si(Li), Low-Energy Ge and Reverse Elec- trode Ge detectors, are usually equipped with a Be or composite carbon cryostat window to take full advantage of their intrinsic energy response. Coaxial Ge detectors are specified in terms of their relative full- energy peak efficiency compared to that of a 3 in. x 3 in. NaI(Tl) Scintillation detector at a detector to source distance of 25 cm. De- tectors of greater than 100% relative efficiency have been fabricated from germanium crystals ranging up to about 75 mm in diameter. About two kg of germanium is required for such a detector. Curves of detector efficiency vs. energy for various types of Ge detectors can be found in the Detector Product Section of this catalog. Figure 1.8 Model 7500SL Vertical Dipstick Cryostat Figure 1.9 Detector Structures and Energy Ranges 1. A.C. Melissinos, Experiments in Modern Physics, Academic Press, New York (1966), p. 178. Charged Particle Detection Phone contact information Benelux/Denmark (32) 2 481 85 30 • Canada 905-660-5373 • Central Europe +43 (0)2230 37000 • France (33) 1 39 48 52 00 • Germany (49) 6142 73820 • Japan 81-3-5844-2681 • Russia (7-495) 429-6577 • United Kingdom (44) 1235 838333 • United States (1) 203-238-2351 For other international representative offices, visit our web site: http://www.canberra.com or contact the CANBERRA U.S.A. office. 6/06 Printed in U.S.A. SILICON CHARGED PARTICLE DETECTORS Silicon Charged Particle detectors have a P-I-N structure in which a depletion region is formed by applying reverse bias, with the resultant electric field collecting the electron-hole pairs produced by an incident charged particle. The resistivity of the silicon must be high enough to allow a large enough depletion region at moderate bias voltages. A traditional example of this type of detector is the Silicon Surface Barrier (SSB) detector. In this detector, the n-type silicon has a gold surface-barrier contact as the positive contact, and deposited aluminum is used at the back of the detector as the ohmic contact. A modern version of the charged particle detector is the CANBERRA PIPS ® detector (Passivated Implanted Planar Silicon). This detector employs implanted rather than surface barrier contacts and is therefore more rugged and reliable than the Silicon Surface Barrier (SSB) detector it replaces. At the junction there is a repulsion of majority carriers (electrons in the n-type and holes in p-type) so that a depleted region exists. An applied reverse bias widens this depleted region which is the sensitive detector volume, and can be extended to the limit of breakdown voltage. Detectors are generally available with depletion depths of 100 to 700 µm. Detectors are specified in terms of surface area and alpha or beta particle resolution as well as depletion depth. The resolution depends largely upon detector size, being best for small area detectors. Alpha resolution of 12 to 35 keV and beta resolutions of 6 to 30 keV are typical. Areas of 25 to 5000 mm 2 are available as standard, with larger detectors available in various geometries for cus- tom applications. Additionally, PIPS detectors are available fully depleted, so that a dE/dx energy loss measurement can be made by stacking detectors on axis. Detectors for this application are sup- plied in a transmission mount, (i.e. with the bias connector on the side of the detector). A chart of the energies of various particles measured at several depletion depths is shown in Table 1.3. Note that even the thinnest detector is adequate for alpha particles from radioactive sources, but that only very low energy electrons are fully absorbed. However, for a detector viewing a source of electron lines, such as conversion electron lines, sharp peaks will be observed since some electron path lengths will lie fully in depleted region. Figure 1.10 shows ranges of particles commonly occurring in nuclear reactions. Table 1.3 Particle Ranges and PIPS Depletion Depth Maximum Particle Energy Depletion Depth (Range) in µm Electron Proton Alpha 100 300 500 700 1000 0.15 0.31 0.45 0.52 0.73 7 15 21 27 33 15 55 85 105 130 Since charge collected from the particle ionization is so small that it is impractical to use the resultant pulses without intermediate amplification, a charge-sensitive preamplifier is used to initially prepare the signal. Figure 1.11 illustrates the electronics used in single-input alpha spectroscopy application. Note that the sample and detector are located inside a vacuum chamber so that the energy loss in air is not involved. LIQUID SCINTILLATORS Two very important beta-emitting isotopes, tritium and 14 C, have very low energy beta rays. These are at 19 and 156 keV respectively, too low to detect reliably with solid scintillators. The liquid scintillation technique involves mixing a liquid scintillator with the sample, and then observing the light pulses with one or more photomultiplier tubes. The efficiency of such a counter is virtually 100% – essentially 4π geometry with no attenuation between source and detector. Pulse processing of the resultant Photomultiplier outputs allows the rejection of cosmic events, and the separation, if desired, of alpha and beta events. The increased sensitivity of the Liquid Scintillation counter, coupled with advances in sample preparation techniques, has led to its increasing use for low-level alpha and beta measurements. Figure 1.10 Range-Energy Curves in Silicon Figure 1.11 ©2006 Canberra Industries, Inc. All rights reserved. Basic Counting Systems Phone contact information Benelux/Denmark (32) 2 481 85 30 • Canada 905-660-5373 • Central Europe +43 (0)2230 37000 • France (33) 1 39 48 52 00 • Germany (49) 6142 73820 • Japan 81-3-5844-2681 • Russia (7-495) 429-6577 • United Kingdom (44) 1235 838333 • United States (1) 203-238-2351 For other international representative offices, visit our web site: http://www.canberra.com or contact the CANBERRA U.S.A. office. 6/06 Printed in U.S.A. PULSE ELECTRONICS The nuclear electronics industry has standardized the signal defi- nitions, power supply voltages and physical dimensions of basic nuclear instrumentation modules using the Nuclear Instrumentation Methods (NIM) standard initiated in the 1960s. This standardiza- tion provides users with the ability to interchange modules, and the flexibility to reconfigure or expand nuclear counting systems, as their counting applications change or grow. CANBERRA is a leading supplier of Nuclear Instrumentation Modules (also called NIM), which are presented in greater detail in Section 1 of this catalog. In the past several years, the industry trend has been to offer modular detector electronics with the multichannel analyzer (MCA) and all supporting instrumentation for spectroscopy with a single detector combined in a compact, stand-alone enclosure. These modular MCAs are smaller, lighter and use less power than the NIM-based counting systems that preceded them. However, their performance is equal to, or greater than, comparable NIM-based systems. CANBERRA is also a leading supplier of these modular detector electronics which are described in the Multichannel Analyzers Sec- tion of this catalog. Depending on the application and budget, NIM or modular electronics may be the best counting equipment solution for the user, and CANBERRA supports both of these form factors with a wide variety of products. Basic electronic principals, components and configurations which are fundamental in solving common nuclear applications are discussed below. PREAMPLIFIERS AND AMPLIFIERS Most detectors can be represented as a capacitor into which a charge is deposited, (as shown in Figure 1.12). By applying detector bias, an electric field is created which causes the charge carriers to migrate and be collected. During the charge collection a small current flows, and the voltage drop across the bias resistor is the pulse voltage. The preamplifier is isolated from the high voltage by a capacitor. The rise time of the preamplifier’s output pulse is related to the collection time of the charge, while the decay time of the preamplifier’s output pulse is the RC time constant characteristic of the preamplifier itself. Rise times range from a few nanoseconds to a few microseconds, while decay times are usually set at about 50 microseconds. Charge-sensitive preamplifiers are commonly used for most solid state detectors. In charge-sensitive preamplifiers, an output voltage pulse is produced that is proportional to the input charge. The output voltage is essentially independent of detector capacitance, which is especially important in silicon charged particle detection (i.e. PIPS ® detectors), since the detector capacitance depends strongly upon the bias voltage. However, noise is also affected by the capacitance. To maximize performance, the preamplifier should be located at the detector to reduce capacitance of the leads, which can degrade the rise time as well as lower the effective signal size. Additionally, the preamplifier also serves to provide a match between the high impedance of the detector and the low impedance of coaxial cables to the amplifier, which may be located at great distances from the preamplifier. The amplifier serves to shape the pulse as well as further amplify it. The long delay time of the preamplifier pulse may not be returned to zero voltage before another pulse occurs, so it is important to shorten it and only preserve the detector information in the pulse rise time. The RC clipping technique can be used in which the pulse is differentiated to remove the slowly varying decay time, and then integrated somewhat to reduce the noise. The unipolar pulse that results is much shorter. The actual circuitry used is an active filter for selected frequencies. A near-Gaussian pulse shape is produced, yielding optimum signal-to-noise characteristics and count rate performance. Figure 1.12 Basic Detector and Amplification Figure 1.13 Standard Pulse Waveforms A second differentiation produces a bipolar pulse. This bipolar pulse has the advantage of nearly equal amounts of positive and negative area, so that the net voltage is zero. When a bipolar pulse passes from one stage of a circuit to another through a capacitor, no charge is left on the capacitor between pulses. With a unipolar pulse, the charge must leak off through associated resistance, or be reset to zero with a baseline restorer. High performance gamma spectrometers are often designed today using Digital Signal Processing (DSP) techniques rather than analog shaping amplifiers. The shaping functions are then performed in the digital domain rather than with analog circuitry. This is discussed later in this section. Typical preamplifier and amplifier pulses are shown in Figure 1.13. The dashed line in the unipolar pulse indicates undershoot which can occur when, at medium to high count rates, a substantial amount of the amplifier’s output pulses begin to ride on the undershoot of the previous pulse. If left uncorrected, the measured pulse amplitudes for these pulses would be too low, and when added to pulses whose amplitudes are correct, would lead to spectrum broadening of peaks in acquired spectra. To compensate for this effect, pole/zero cancel- lation quickly returns the pulse to the zero baseline voltage. The bipolar pulse has the further advantage over unipolar in that the zero crossing point is nearly independent of time (relative to the start of the pulse) for a wide range of amplitudes. This is very useful in timing applications such as the ones discussed below. However, the unipolar pulse has lower noise, and constant fraction discrimina- tors have been developed for timing with unipolar pulses. For further discussions on preamplifier and amplifier characteristics, please refer to each applicable module’s subsection. Figure 1.14 Multichannel Analyzer Components with Analog Signal Processing PULSE HEIGHT ANALYSIS AND COUNTING TECHNIQUES Pulse Height Analysis may consist of a simple discriminator that can be set above noise level and which produces a standard logic pulse (see Figure 1.13) for use in a pulse counter or as gating signal. However, most data consists of a range of pulse heights of which only a small portion is of interest. One can employ either of the following: 1. Single Channel Analyzer and Counter 2. Multichannel Analyzer The single channel analyzer (SCA) has a lower and an upper level discriminator, and produces an output logic pulse whenever an input pulse falls between the discriminator levels. With this device, all voltage pulses in a specific range can be selected and counted. If additional voltage ranges are of interest, additional SCAs and counters (i.e. scalers) can be added as required, but the expense and complexity of multiple SCAs and counters usually make this configuration impractical. If a full voltage (i.e. energy) spectrum is desired, the SCA’s discrimi- nators can be set to a narrow range (i.e. window) and then stepped through a range of voltages. If the counts are recorded and plotted for each step, an energy spectrum will result. In a typical example of the use of the Model 2030 SCA, the lower level discriminator (LLD) and window can be manually or externally (for instance, by a computer) incremented, and the counts recorded for each step. However, the preferred method of collecting a full energy spectrum is with a multichannel analyzer. The multichannel analyzer (MCA), which can be considered as a series of SCAs with incrementing narrow windows, basically consists of an analog-to-digital converter (ADC), control logic, memory and display. The multichannel analyzer collects pulses in all voltage ranges at once and displays this information in real time, providing a major improvement over SCA spectrum analysis. Figure 1.14 illustrates a typical MCA block diagram. An input energy pulse is checked to see if it is within the selected SCA range, and then passed to the ADC. The ADC converts the pulse to a number proportional to the energy of the event. This number is taken to be the address of a memory location, and one count is added to the contents of that memory location. After collecting data for some period of time, the memory contains a list of numbers corresponding to the number of pulses at each discrete voltage. The memory is accessed by a host computer which is responsible for spectrum display and analysis as well as control of the MCA. Depending on the specific model MCA, the host computer may be either a dedicated, embedded processor or a standard off-the-shelf computer. PULSE HEIGHT ANALYSIS WITH DIGITAL SIGNAL PROCESSORS Today’s high performance Multichannel Analyzer systems are designed using Digital Signal Processing (DSP) techniques rather than the traditional analog methods. DSP filters and processes the signals using high speed digital calculations rather than manipula- tion of the time varying voltage signals in the analog domain. The preamplifier signal first passes through an analog differentiator, then is delivered to a high speed digitizing ADC (Figure 1.15). The output of the ADC is a series of digital values that represent the differentiated pulse. Those signals are then filtered using high-speed digital calculations within the Digital Signal Processor. For optimal speed and accuracy in signal processing, a trapezoidal filter algorithm is deployed in the DSP implementation. Trapezoidal filtering has been shown to allow for the highest throughput performance with the least degradation of spectral resolution. Addition- ally, the DSP based design is intrinsically more stable, resulting in better performance over a range of environmental conditions. COUNTERS AND RATEMETERS Counters and ratemeters are used to record the number of logic pulses, either on an individual basis as in a counter, or as an aver- age count rate as in a ratemeter. Counters and ratemeters are built with very high count rate capabilities so that dead times are mini- mized. Counters are usually used in combination with a timer (either Figure 1.15 Multichannel Analyzer Components with Digital Signal Processing Figure 1.16 NaI Detector and Counter/Timer with Alarm Ratemeter built-in, or external), so that the number of pulses per unit of time are recorded. Ratemeters feature a built-in timer, so that the count rate per unit of time is automatically displayed. Whereas counters have an LED or LCD for the number of logic pulses, ratemeters have a mechanical meter for real-time display of the count rate. Typically, most counters are designed with 8-decade count capacity and offer an optional external control/output interface, while ratemeters are designed with linear or log count rate scales, recorder outputs and optional alarm level presets/outputs. Additional information may be found in the Counters and Ratemeters Introduction. SIMPLE COUNTING SYSTEMS As related above, pulse height analysis can consist of a simple single channel analyzer and counter, or a multichannel analyzer. Gen- erally, low resolution/high efficiency detectors (such as proportional counters and NaI(Tl) detectors) are used in X ray or low-energy gamma ray applications where only a few peaks occur. An example of a simple NaI(Tl) detector-based counting system of this type is illustrated in Figure 1.16. In this configuration, a Model 2015A Amplifier/SCA is used to gen- erate a logic pulse for every amplified (detector) pulse that falls within the SCA’s “energy window”. The logic pulse is then used as an input to the Model 512 Counter/Timer which provides the user with a choice of either preset time or preset count operation. The Model 512 is equipped with an RS-232 interface, which enables it to be controlled and read out to a computer for data storage or further analysis. Alternatively, Model 1481LA Linear/Log Ratemeter is used as the counter, with an alarm relay that will trigger if the count rate exceeds a user preset value. Although counters are still used in some applications, most of today’s counting systems include a multichannel analyzer (MCA). Besides being more cost effective than multiple SCA-based systems, a MCA-based system can provide complete pulse height analysis such that all nuclides, (i.e., even those not expected), can be easily viewed and/or analyzed. NaI(Tl) DETECTORS AND MULTICHANNEL ANALYZERS The need for a single-input Pulse Height Analysis system for use with a Sodium Iodide detector is served most simply by a photomultiplier tube (PMT) base MCA such as the uniSpec (Figure 1.17). The uni- Spec MCA includes a high voltage power supply, preamplifier, amplifier, spectrum stabilizer and ADC in addition to its MCA functions, and thus, there is no need for any NIM modules or a NIM Bin. All of Figure 1.18 HPGe Detector and Analog MCA Configuration this capability is provided in an enclosure no larger than a standard tube base preamplifier, and the computer interface is via a USB port on the host computer or a USB hub. Further technical discussions concerning multichannel analyzers and multichannel analysis systems (including spectroscopy software) may be found in the Multi- channel Analyzers and Counting Room Software sections. GERMANIUM DETECTORS AND MULTICHANNEL ANALYZERS A typical analog HPGe detector-based gamma spectroscopy system consists of a HPGe detector, high voltage power supply, preamplifier (which is usually sold as part of the detector), amplifier, ADC and multichannel analyzer. As will be discussed in more detail later, DSP configurations replace the amplifier and ADC with digital signal processing electronics. The analog system components are available in several different types, allowing the system to be tailored to the precise needs of the application and the available budget. For example, low-end amplifiers such as the Model 2022 offer basic capabilities, but users with higher count rate or resolution requirements may consider the Mod- el 2026 or 2025 with Pileup Rejection/Live Time Correction (PUR/ LTC) feature and both Gaussian and triangular shaping. Similarly, the ADC chosen for a system including a 556A NIM MCA could be either an economical Wilkinson ADC like the Model 8701 or a faster Fixed Dead Time (FDT) ADC like Model and 8715. For more Figure 1.17 NaI Detector and MCA Configuration [...]... there are trade-offs between background continuum and lead X- ray peaks The graded liners typically used to suppress the lead X rays (75-85 keV) consist of 0.5 to 1.5 mm thick layers of cadmium and copper The cadmium is an effective filter for lead X rays while the copper attenuates the cadmium X rays and prevents personnel exposure to the toxic cadmium This graded liner has the undesirable effect of... 2000 Section 1 Sample Changer Based Gamma Analyst Genie-ESP or Apex (Gamma) Section 1 (HPGe) Portable/Battery Powered InSpector 2000 Genie 2000 Section 1 (NaI) required Portable/No AC Power uniSpec InSpector 1000 Genie 2000 Genie 2000 optional Section 1 Multi-Station/Multi-Input or Single Station/Multi-Input Alpha Analyst Genie-ESP/Apex-Alpha Client/Server or Apex-Alpha Desktop Section 1 Single Station/Limited... the K-shell X rays vary together with 210 Pb levels (the beta obviously excites the K-shell X rays) However, the continuum differences are fairly small, even with the roughly 3:1 difference in 210Pb content Above 500 keV, there is no difference in backgrounds Since the graded liner Now the 46.5 keV gamma ray from 210 Pb is readily stopped by the graded liner used to suppress lead K-shell X rays However,... interfering gammas from those fission products may very well overwhelm the weak gammas emitted from the fissile material, making neutron counting the only viable method available for performing the assay In general neutron and gamma techniques are complementary HRGS may provide relative isotopic information, for example, and the neutron assay bulk non destructive quantification Phone contact information Benelux/Denmark... and counts due to “accidental coincidences” Once one neutron has been detected, the probability of detecting another neutron from the same fission decreases approximately exponentially with time according to the following equation: P(t) = exp(–t/t d ) where P(t) = Probability of detecting coincidence neutrons in time t td = die-away time of the moderated detector assembly The die-away time is the characteristic... the integrator, typically in one or two milliseconds If the incoming energy rate (count rate X energy/count) produces a current that exceeds the capability of the resistor to bleed it off, the output will increase until, in the extreme, the preamplifier saturates and ceases to operate This limit occurs at approximately 200k MeV/s The saturated condition remains until the count rate is reduced The saturation... of the time delay between the two events These two approaches are used in gamma- gamma or particle -gamma coincidence measurements, positron lifetime studies, decay scheme studies and similar applications, and are titled coincidence or timing measurements A coincidence system determines when two events occur within a certain fixed time period However, in practice it’s not possible to analyze coincidence... end coaxial geometry, to assure that the entire front face is active Advances in electronics technology have dramatically lowered the cost of MCAs, so that today, it is frequently more effective to use multiple complete MCA systems (or the Multiport II) in place of a Multiplexer LOW LEVEL GAMMA RAY COUNTING Large volume HPGe detectors have become dominant over other detector types for low level gamma. .. with 0.5 mm of cadmium but this will stop only about 70% of the lead X rays One mm of tin will stop about 95% of the lead X rays With an additional 1.5 mm of copper, the total attenuation of lead X rays in the Canberra shields is about 98.5% Another disadvantage of cadmium is high cross-section for neutrons from cosmic radiation For example, the 113Cd (η, γ) 114Cd reaction results in a prominent background... from 40% coaxial detector (1) unshielded (2) shielded with standard cryostat and (3) shielded with Ultra Low-background cryostat Table 1 - Model List For Ultra Low-Background Cryostat Options Cryostat Type Base Cryostat Hardware Option Ultra Low-Background Material Vertical Dipstick U-Style 7500SL 7915-30 RDC SL ULB-GC (Coaxial) ULB-GR (REGe) ULB-GL (LEGe) ULB-GW (Well) For example a coaxial detector

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