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Basic Counting Systems -

Pulse Electronics

The nuclear electronics industry has standardized the signal definitions, power supply voltages and physical dimensions of basic nuclear instrumentation modules (NIM). The standardization 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 (NIM). 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 1.13). 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.

Figure 1.13: Basic Detector
and Amplification

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.

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.

Typical preamplifier pulses are shown in Figure 1.14.

Figure 1.14: Standard Pulse Waveforms

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 cancellation 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 discriminators have been developed for timing with unipolar pulses.

For further discussions on preamplifier and amplifier characteristics, please refer to each applicable module’s subsection.

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 1.14) 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:

• Single Channel Analyzer and Counter
• 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 discriminators 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 1.15 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 display reads the memory contents vs. memory locations, which is equivalent to number of pulses vs. energy.

Figure 1.15.
Multichannel Analyzer Components

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 average count rate as in a ratemeter. Counters and ratemeters are built with very high count rate capabilities so that dead times are minimized. Counters are usually used in combination with a timer (either 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 a 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 section.

Miscellaneous Units

Various pulse processing functions have been incorporated into NIM units, such as linear gates, pulse generators, pulse stabilizers, etc. Many of these are described in the following sections and in the introduction to each Nuclear Instrumentation Modules (NIM) Section.

Simple Counting Systems

As related above, pulse height analysis can consist of a simple single channel analyzer and counter, or a multichannel analyzer. Generally, 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.

Figure 1.16: NaI Detector and Counter/Timer with Alarm Ratemeter

In this configuration, a Model 2015A Amplifier/SCA is used to generate 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 2071A Counter/Timer which provides the user with a choice of either preset time or preset count operation. The Model 2071A may be equipped with an optional Model 207X-03 EIA Interface, which enables the Model 2071A to be read out to a printer, or 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 with Sodium Iodide detector is best served by a PC-card MCA, such as the AccuSpec NaI/Plus (Figure 1.17).

Figure 1.17: AccuSpec/NaI Plus
MCA Configuration

This single plug-in board includes a High Voltage Power supply, Amplifier, and ADC in addition to its MCA functions, and thus, there is no need for additional modules or a NIM Bin. Further technical discussions concerning multichannel analyzers and multichannel analysis systems (including spectroscopy software) may be found in Multichannel Analyzers and Advanced Spectroscopy Software sections.

PIPS Detectors and Multichannel Analyzers

Alpha spectroscopy measurements of low-level samples require long counting times. A large area PIPS detector, when configured with a Canberra alpha spectrometer and multichannel analyzer, provides a high resolution, low background, counting system that will satisfy a multitude of alpha spectroscopy applications.

An example of a single chamber alpha spectroscopy system (that can easily be upgraded) is illustrated in Figure 1.12. Note that the Model 7401 Alpha Spectrometer is a complete, self-contained, 2-wide NIM module that contains a vacuum chamber, vacuum gage, detector bias supply, preamplifier/amplifier, SCA, counter/timer and pulser for setup and test. Multiple Model 7401 Alpha Spectrometers can be configured with a vacuum system that allows individual vacuum chambers to be opened and loaded without affecting the vacuum or data acquisition of the other spectrometers.

However, where numerous samples are counted simultaneously, it may be more cost effective and user efficient to select a system based on the Alpha Analyst (Figure 1.18).

Figure 1.18 Example Large Scale Alpha Spectroscopy System

This turn-key system supports multiple detectors in a comprehensive software environment featuring full computer control of all vacuum elements and acquisition electronics. To learn more about Canberra’s Alpha Analyst, click here.

Germanium Detectors and Multichannel Analyzers

A typical 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, analog-to-digital converter and multichannel analyzer. Figure 1.19 illustrates a simple gamma spectroscopy system. This configuration shows NIM electronics for the front end, allowing selection of the optimal spectroscopy amplifier. Canberra offers traditional ‘manually-operated’ NIM modules, as well as a selection of computer-controlled front ends.

Figure 1.19.
HPGe Detector and MCA

For higher count rate applications, it is necessary to use an additional circuit to reject pileup pulses that can distort the spectrum. Pileup Rejection/Live Time Correction (PUR/LTC) inspects both the leading edge and the trailing edge of the pulse and can discriminate between two events separated by less than 0.5 microseconds. Since these pileup pulses are rejected, the ADC live time must be lengthened to properly compensate for time the system was unable to process pulses. Virtually all current Canberra ADCs and MCAs (including AccuSpec, InSpector, and AIM) provide signal paths for pulse Pileup Rejection/Live Time Correction as illustrated in Figure 1.20.

Figure 1.20.
HPGe Detector with PUR/LTC

LEGe and Si(Li) Detectors with Multichannel Analyzers

Low Energy Germanium (LEGe) and Si(Li) detectors require special circuitry to provide the long time constants required in the amplifier to achieve maximum resolution, and to properly handle the pulsed optical feedback signals of their preamplifiers. Although several Canberra amplifiers are suitable, the best resolution will be obtained with the Model 2025 AFT Research Amplifier. Besides allowing the user to select a long shaping time constant, the Model 2025 features an enhanced baseline restorer which is ideal for pulsed optical feedback preamplifiers.

In high count rate applications, the long time constants contribute to Pulse Pileup. The Model 2025 contains a built-in Live Time Corrector/Pileup Rejector to prevent these inaccuracies. A typical example of a LEGe or Si(Li) based system is illustrated in Figure 1.21. Note that this system also includes an optional Model 1786A Detector LN₂ Monitor to prevent accidental damage to the detector caused by running out of liquid nitrogen.

Figure 1.21.
LEGe or Si(Li) Detector and MCA

Multiple Input Systems

Traditionally, Mixer/Routers, or Multiplexers, were employed to allow several detectors to be counted in one MCA (with one ADC), as shown in Figure 1.22. Advances in computer technology have dramatically lowered the cost of MCAs and memory, so that, today, it is frequently more effective to use multiple MCAs in place of a Mixer/Router. The Mixer/Router configuration has a major drawback in that a single ADC processes signals from all detectors, and thus the count rate on the individual detectors must be relatively low to avoid excessive pileup. Additionally, a Mixer/Router must allocate the memory of the MCA among its inputs, which decreases the number of channels available for an individual channel. Within these constraints, Mixer/Routers can be quite efficient for applications such as low-level environmental alpha spectroscopy, in which multiple low-intensity inputs are collected in MCA memory segments of 512 channels or less.

Figure 1.22.
Multiple Input System

Low Level Gamma Ray Counting

Large volume HPGe detectors have become dominant over other detector types for low level gamma ray spectroscopy because of their inherently good resolution and linearity. It is only in the analysis of single radionuclides that NaI(Tl) detectors can compare in sensitivity with HPGe detectors. Since the majority of all gamma spectroscopy applications require the analysis of more complex, multi-radionuclide samples, the following discussion will be limited to the application of HPGe detectors to low level counting.

The sensitivity of a HPGe spectrometer system depends on several factors, including detector efficiency, detector resolution, background radiation, sample constituency, sample geometry and counting time. The following paragraphs discuss the role these factors play in low level gamma ray counting.

1. Efficiency
   Generally, the sensitivity of a HPGe system will be in direct proportion to the detector efficiency. HPGe detectors are almost universally specified for efficiency relative to a 3 in. NaI(Tl) at 25 cm detector-to-source distance at 1.33 MeV, and from this benchmark one may roughly impute the efficiency at lower energies. However, for the customer who is counting weak samples with lower gamma energies, for instance 100-800 keV, the following subtle considerations to the detector design are important to system performance:

2. Resolution
   Generally, the superior resolution of a HPGe detector is sufficient enough to avoid the problem of peak convolution, (i.e., all peaks are separate and distinct). The sensitivity of a system improves as the resolution improves because higher resolution means that spectral line widths are smaller, and fewer background counts are therefore involved in calculating peak integrals.
   
   Since the sensitivity is inversely related to the square root of the background, that is,

3. Background Radiation and Sample Constituency
   Interfering background in gamma spectra originates either from within the sample being counted (Compton-produced) or from the environment. If the sample being analyzed has a high content of high-energy gamma emitting radioisotopes, the Compton- produced background will easily outweigh the environmental background. For extremely weak samples, the environmental background becomes more significant. Obviously, massive shielding will do little to improve system sensitivity for low energy gamma rays in the presence of relatively intense higher energy radiation. However, Compton-suppression can be very effective in reducing this background.

4. Sample Geometry
   An often overlooked aspect of HPGe detector sensitivity is the sample geometry. For a given sample size (and the sample size should be as a large as practicable for maximum sensitivity), the sample should be distributed so as to minimize the distance between the sample volume and the detector itself.
   
   This rules out analyzing "test tube" samples with non-well type detectors, or "large area flat samples" with standard detectors. It does rule in favor of using re-entrant or Marinelli-beaker-type sample containers, which distribute part of the sample around the circumference of the detector.

Germanium Detectors with Inert Shields

There are many different types of shield designs that are available, and because of the difficulty in determining the background contribution of the materials used in a given shield, it is difficult to assign performance levels to various types of shields. However, some criteria for shield designs have evolved over the years, such as:

1. The shield should not be designed to contain unnecessary components like the Dewar. It will only contribute to increased background if it is within the walls of the shield, as well as unnecessarily increase the shield’s size, weight and cost.
   
2. The detector should be readily installed and removable from the shield.
   
   Pity the person who has to move lead bricks (at 12 kg each) to disengage a HPGe detector. A HPGe detector and shield system should have a liquid nitrogen transfer system to avoid removing the detector for the weekly filling.
   
3. Sample entry should be convenient to the operator.
   
4. The shield should accommodate a variety of sample sizes and configurations.
   
   The HPGe detector should be located in the center of the shield so as to minimize scatter from the walls. In this position, the shield must accommodate the largest sample that is anticipated. Also, sample placement should be accurately repeatable and easily verified by the operator.

The shield design that has all these features and is moderately priced is the Canberra Model 747 Lead Shield illustrated in Figures 1.23 and 1.24.

Figure 1.23 Detector located in center of chamber without requirement for extended end-cap

Figure 1.24 Model 747 Lead Shield

The performance of the shield using a Canberra HPGe detector is given below:

Shield Specs
Inside Dimensions	28 cm dia. x 40.5 cm high
Wall Thickness	10 cm
Material	Low Background Lead

HPGE Specs
Rel. Efficiency	12%
Resolution	1.95 keV FWHM at 1.33 MeV 0.90 keV FWHM at 1.22 keV

Background Count

2.25 counts/second in the 50 keV-2.7 MeV range

Table 1.4 lists the sensitivities of several single radioisotopes, assuming a counting time of 50 000 seconds, a 50% error and a detector-to-point-source distance of 1 cm.

Table 1.4 Radioisotope vs. Sensitivity
Radionuclide	Energy in keV	Sensitivity in pC
57Co	122	2
139Ce	165	3
137Cs	662	6
60Co	133	10

Low Background Cryostats

The design or configuration of the cryostat is another factor in system performance. Some cryostat/shield designs do not prevent streaming from the outside environment, nor do they provide self-shielding from their own relatively hot components. Through an improper choice of material types and/or thicknesses, the cryostat may appreciably contribute to the background. Canberra has developed sources for select, low-background, materials, and has invested in the design and fabrication of low-background cryostats, as described in the Introduction to the Cryostats and Accessories Section.

HPGe Compton Suppression Spectrometer

When the ultimate in low level counting is required, a Compton Suppression Spectrometer, in conjunction with an appropriate low-background shield/cryostat design, is the answer. In this configuration, the HPGe detector is surrounded by an active NaI(Tl) or plastic scintillation guard detector (also known as an annulus detector), with the electronics configured in an anticoincidence counting mode. The Compton continuum, which is primarily caused by gamma rays which sustain one or more inelastic collisions and escape (i.e. scatter out of) the germanium detector material without imparting their full energy, can lead to concealment of low activity peaks. Since this is undesirable in low level counting applications, a Compton Suppression Spectrometer can be used to gate (i.e. turn off) data acquisition whenever one of the incompletely absorbed photons escapes the germanium detector and is "seen" by the annulus detector. When acquisition is complete, the resultant spectrum only contains peaks attributed to gamma rays which have imparted their full energy within the detector material.

It should be pointed out that some radioisotopes (those having coincident gamma rays) such as ⁶⁰Co, will not be analyzed properly by the anticoincidence spectrum from a Compton Suppression System. Therefore, two spectra are usually obtained from such a spectrometer - one in the anticoincidence mode, and the other in the normal (ungated) mode.

Figure 1.25 illustrates a typical example of a Compton Suppression System.

Figure 1.25.
Compton Suppression system

One type of annulus has six (6) 5.08 cm (2-inch) diameter photomultiplier tubes (PMTs) on one end, and a 7.62 cm (3-inch) diameter NaI(Tl) plug with one PMT (which is operated in parallel with the other PMT) on the other end. A simpler type of annulus detector uses a 15.24 cm (6-inch) diameter NaI(Tl) well detector on a single PMT. In either configuration, the annulus must be large enough to allow the insertion of the HPGe detector’s endcap along with the sample.

While some endorse the use of a fairly complex Timing Chain to derive the anti-Compton gate signal, Canberra has found that the simplified circuit shown in Figure 1.25 yields equivalent results (See the Compton Suppression Made Easy Application Note). The "Incoming Count Rate" signals from the Spectroscopy Amplifiers are checked for coincidence, and, if it exists, the 2040 Coincidence Analyzer’s output is used as an anti-coincidence input to the ADC’s Gate. When coincidence occurs, this gate "turns off" the delayed unipolar signal from the Spectroscopy Amplifier. Typical Compton Suppression Spectrometer results are illustrated in Figure 1.26.

Figure 1.26 Ge Spectra with
Compton Suppression

It can be seen that the ‘figure of merit’ - the value of the ¹³⁷Cs peak at 662 keV divided by the average contents of the Compton continuum (the energy range 358-382 keV) - is on the order of 1000:1.

High Count Rate Gamma Ray Systems

High count rate applications require special techniques to assure good resolution and/or good throughput. In general, "high count rate" is used to refer to incoming count rate, that is, the number of events seen by the detector. The term "throughput rate" may be of more interest to the researcher, being a measure of the rate at which the system can accurately process these incoming counts.

In high count rate HPGe detector applications, problems such as the loss of resolution, excessively long counting times, erroneous peak to background ratios, inaccurate counting statistics or system shutdown due to overload and saturation begin to appear. In some experiments, the solution to these problems merely lies in reducing the incoming count rate to the detector, or by employing electronics which inhibit the processing of pulses through the electronics when events are occurring so fast that they are overlapping (pulse pileup). In this latter solution, system throughput will of course be reduced, but parameters such as resolution will be enhanced. Table 1.5 indicates the throughput limitations of the major components of a spectroscopy chain. Note that the term "energy rate limited" refers to the fact that the component’s performance is not only affected by the incoming count rate, but by the relative energy (amplitude) of the incoming counts as well. Each element in the chain can be optimized for high count rate performance.

Table 1.5.
Major System Components and their Throughput Limitations

The Detector

For the detector itself, the charge collection time is the limiting factor, and this parameter is a function of the detector geometry - when a photon interaction takes place, charge carriers in the form of holes and electrons are produced, and the time taken for these carriers to be swept to the P and N electrodes of the detector is the time for full energy collection. In a Germanium detector, this time is a function of detector size, as the charge carriers travel about 0.1 mm/sec. As the charge collection time increases, the Amplifier must take a longer time to process the signal and develop its linear pulse, or else not all of the incident energy will be reflected in that pulse ("ballistic deficit"). Thus, larger detectors require longer amplifier time constants, or more sophisticated peak shapes.

Some ways to address high count rate in the detector include moving the detector farther away from the source, or collimating the detector - which in both cases reduces the number of events seen by the detector - or using a detector of less efficiency. The detector in the latter case will ‘see’ fewer events, and furthermore will have a lower charge collection time.

The Preamplifier

Most Germanium detectors in use today are equipped with RC-feedback, charge sensitive preamplifiers. In the RC-feedback preamplifier, a feedback resistor discharges 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 limit is dependent on both energy and count rate and is usually specified in terms of the "energy/rate limit". The energy/rate limit can be increased by lowering the value of the feedback resistor, but this in turn allows more noise to pass through the preamplifier, resulting in a degradation in resolution.

When a Coaxial Germanium detector is used in applications requiring high throughput, the Model 2101 Transistor Reset Preamplifier (TRP) is favored over traditional RC feedback Preamplifiers. The higher cost of the TRP is justified by its much higher energy rate capacity, an enhancement obtained by replacing the Feedback Resistor of a typical RC feedback preamplifier with a special reset circuit. This circuit monitors the dc level of the preamplifier and discharges the feedback capacitor whenever the output reaches a predetermined reset threshold. At moderate to high count rates (i.e. above 20 000 cps), the absence of the feedback resistor and its attendant noise and secondary time constant contributions lead to:

• Lower preamplifier noise contributions.
  
• Inherently better resolution and reduced spectrum broadening vs. count rate.
  
• Elimination of the need for pole-zero cancellation.

• Elimination of ‘lock-up’ due to saturation.

Figure 1.27 illustrates the throughout performance of the two preamplifier styles.

Figure 1.27 Throughput vs. Count Rate: Throughput Optimization

Although the Model 2101 TRP virtually never shuts down due to saturation, its reset process and the amplifier overload which it causes does induce intervals of dead time into the counting system. The Model 2101 has been designed with a small Charge Gain (50 mV/MeV) and a wide Dynamic Range (4 V) to significantly reduce the dead time due to resets in comparison to competitive units.

The Amplifier

The function of the spectroscopy amplifier is to filter the signal for optimum signal to noise ratio and to provide gain, by first differentiating the preamplifier output to provide a rapid return to the baseline reference, and then integrating this signal to capture the energy information. For the best resolution performance, the integration stage produces a Gaussian waveform, and the measure of the width of this pulse may be expressed in terms of its time constant. If the time constant is too short, the amplifier will allow more noise to pass, thus degrading resolution.

Additionally a short time constant can fail to give enough time for the entire charge collection to take place in the detector (ballistic deficit), which will also affect resolution. On the other hand, a longer time constant lengthens the output pulse of the amplifier (output pulse width is roughly equal to 7.3 x Time Constant) and allows more pileup effects. Since pileup events - one pulse riding on another - carry invalid information, they are typically gated off from the amplifier output. Thus, decreasing the time constant degrades resolution, and increasing it decreases throughput. Figure 1.28 shows the effect of time constant on throughput and resolution:

Figure 1.28
Resolution vs. Throughput Tradeoffs for a Standard Spectroscopy Amplifier

In practice, Coaxial Germanium detectors are usually not operated below 2 ms Gaussian Shaping, and, more typically, at
4 ms shaping to optimize resolution.

A technique called Gated Integration (GI) can be used to allow the use of short shaping time constants. The GI Amplifier integrates the entire unipolar signal, providing a properly scaled Linear Output signal with a relatively short time constant, while avoiding the severe impact on resolution that would be seen with a Gaussian pulse of the same width.

While the Gated Integrator, with shaping time constants on the order of 0.25 ms, allows higher throughput, it does introduce a certain amount of noise, and thus cannot achieve the same resolution as a standard Gaussian output at, for example, a shaping time of 4 ms. However, this slight and constant degradation in resolution may be a desirable tradeoff in terms of throughput, as evidenced by the plots of count rate and resolution in Figure 1.29.

Figure 1.29 The Gated Integrator vs.
a Spectroscopy Amplifier

The ADC

Canberra offers two styles of ADC’s - Wilkinson and Fixed Dead Time (FDT). The Wilkinson technique achieves better specifications for differential and integral linearity, and hence resolution, but its throughput is dependent not only on the count rate but by the energy, or converted channel number, of the counts. For spectra with conversion gain of 1024 channels or less, the Wilkinson ADC can outperform most FDT ADC’s available today, with conversion times of typically 5-10 ms. However, at higher conversion gains, the FDT ADC’s will have a decidedly faster conversion time, and thus higher throughput. The Canberra Model 8715 ADC, an FDT ADC with a conversion time of 800 ns, clearly offers the best performance up to its conversion limit of 8K channels.

In performance tests, the Model 8715 FDT ADC was found to be so fast that it actually adds NO dead time to spectroscopy systems using traditional Gaussian or triangular shaping amplifiers, and less than 800 nanoseconds to systems using Gated Integrator amplifiers.

Summarizing, the combination of the RC Preamplifier, Model 2025 AFT Research Amplifier, and Model 8701 ADC provide optimum resolution at incident counts rates less than 20 000 cps, but the combination of Model 2101 TRP, Model 2024GI Amplifier, and Model 8715 FDT ADC will yield the highest throughput available - up to 70 000 cps, with only a minor tradeoff in resolution, as seen in Figure 1.29.

Pulse Pileup Rejection

As we saw earlier, pulse pileup occurs when a new pulse from the preamplifier reaches the spectroscopy amplifier before the amplifier has had a chance to finish properly processing the previous pulse. In such cases, an Amplifier/ADC combination with Pile Up Rejection/Live Time Correction (PUR/LTC) has the ability to (a) inhibit the ADC from processing any invalid, composite, pulses, and (b) turn off the Live Time Clock, which is measuring ‘real’ collect time, until the next valid pulse is received at the Amplifier. Typically, this clock is disabled during the time when the Amplifier and ADC are processing a pulse, and thus records the time when the system is ‘ready’ to receive an input pulse.

Digital Signal Processor

Historically, analog devices have been employed to perform the function of the amplifier in Pulse Height Analysis systems. As noted earlier, this function is not as much amplification as it is processing or shaping. The detector signal is processed, shaped, and filtered by the amplifier and then digitized by the ADC at the end of the processing chain. The selection of the output pulse shape and its associated time constants are designed to maximize the signal-to-noise ratio for optimized resolution and reduced sensitivity to ballistic deficit, while providing the maximum throughput consistent with the resolution requirements of the application. With advances in high speed digital hardware, it is now possible to design and implement digital pulse processors to perform the operations previously only performed by an analog amplifier. With digital processing, filtering functions can be employed that are not possible with conventional analog signal processing.

In Canberra’s Digital Pulse Processing system, the detector/preamplifier signal is digitized much earlier in the signal processing chain. Subsequent processing, filtering, baseline restoration, and pileup rejection are performed digitally, and the results are transferred directly to the MCA histogram memory for viewing and analysis. This system, introduced in 1997, has been shown to provide resolution and throughput performance well in excess of commercially available analog systems, as well as increased precision and repeatability.

Loss Free Counting Applications

The correction of the Live Time Clock as described above, effectively extending the counting time to account for those periods when the system could not accept an input, is adequate for most samples, in particular those for which the count rate is relatively constant. However, for short Half-Lived samples, or samples whose constituents change (as in a flow monitoring application), this method will not be accurate. In addition, even if the "counts per unit time" are accurate using the traditional method for dead time correction, the "real" counting time will have been extended by an amount equal to the dead time, which may in fact increase the actual collection time to an undesirable length.

The principal goal of LFC is to insure that at the end of any data acquisition interval, the electronics have accumulated all of the events that occurred regardless of any dead time that may have been present in the system. LFC is based on the concept that by adding "n" counts per event to an MCA’s channel register, rather than digitizing and storing a single count at a time, a "zero dead time" energy spectrum can be accumulated that assures all counts are included in the spectrum. Assuming that "n" is correctly derived, ("n" should equal "1" plus a "weighting factor" representing the number of events that were lost since the last event was stored), and the data is truly random in nature, the concept is statistically valid. The factor "n" is derived on a continuous basis by examining the state of the Amplifier and ADC every 200 ns. The proportion of time during which the Amplifier and ADC are processing a pulse provides a measure for the magnitude of the weighting factor "n", which is updated every 20 ms. Loss free counting requires that the MCA support "add-n" or multiple "add-one" data transfer; consult the factory for details.

Unfortunately, counting statistics in a Loss Free Counting system cannot be calculated from the corrected spectrum. One basic assumption used by all peak fitting algorithms is that of Poisson counting statistics. That is, the uncertainty of the counts is proportional to the square root of the number of counts. While this assumption is true for traditional "add-1" front-ends, it is not true of the "add-n" Loss Free Counting front-end. This assumption is especially poor as the weighting factor becomes large. To properly quantify the uncertainty in each channel’s contents, the peak fitting program must have access to both the corrected "add-n" and the uncorrected "add-1" spectra. Therefore, Canberra also offers a "Dual-LFC" hardware option for the Model 599 which allows the collection of both of these spectra so that statistically correct peak filling can occur. Note that the correction software for the "Dual-LFC" system is only available for VMS-based Genie Systems.