Basic Counting Systems

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PULSE ELECTRONICSThe nuclear electronics industry has standardized the signal definitions, power supply voltages and physical dimensions of basic nuclear instrumentation modules using the nuclear Instrumentation Methods (nIM) standard initiated in the 1960s. This 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 (also called nIM). 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. 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 AMPLIFIERSMost 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 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.

Figure 1.14 Multichannel analyzer Components with analog Signal Processing

PULSE HEIGHT ANALYSIS AND COUNTING TECHNIQUESPulse 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 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 1.14 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 PROCESSORSToday’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 manipulation 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. additionally, the DSP based design is intrinsically more stable, resulting in better performance over a range of environmental conditions.

COUNTERS AND RATEMETERSCounters 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.

Figure 1.15Multichannel analyzer Components with Digital Signal Processing

2015B

Figure 1.16 naI Detector and Counter/Timerwith alarm ratemeter

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 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 SYSTEMSas 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.

In this configuration, a Model 2015B 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 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 ANALYZERSThe 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 Osprey (Figure 1.17). The Osprey MCa includes a high voltage power supply, preamplifier, and DSP electronics in addition to its MCa functions,

Figure 1.18 HPGe Detector and analog MCa Configuration

and thus, there is no need for any nIM modules or a nIM Bin. all of this capability is provided in an enclosure no larger than a standard tube base preamplifier, and the computer interface is via a USB or ethernet port on the host computer or a network hub.

GERMANIUM DETECTORS AND MULTICHANNEL ANALYZERSa 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 Model 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 556B 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 information about selecting specific modules, refer to the introduction sections for those specific components.

802-3x3 Osprey

Osprey MCA with Genie 2000 software

USB

Figure 1.17 naI Detector and MCa Configuration

For applications requiring security of the signal processing, CanBerra offers a variety of computer controlled electronics which require access via a host computer, rather than unprotected front panel for adjustment. For example, the aIM/ICB nIM family is a network based, computer controlled signal processing line that can be controlled remotely by a Genie 2000 or apex-Gamma spectroscopy workstation.

Spectroscopy systems based on Digital Signal Processing (DSP) have been widely accepted as the state of the art. In a DSP based system, the amplifier and aDC are replaced by a set of digital circuits which implement the filtering functions in high speed digital calculations. CanBerra offers several DSP based products, all of which offer superior environmental stability, higher count rate throughput performance and better resolution over a range of count rate conditions. Lynx, DSa-1000, Osprey and the InSpector 2000 all employ this advanced DSP technology.

Figures 1.18, 1.19 and 1.20 show several of the available Germanium Detectors/MCa configurations. Optional Ln2 Monitors, Level alarms, and Control Systems are available for most types of detectors.

LEGe AND Si(Li) DETECTORS WITH MULTICHANNEL ANALYZERSLow 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 reset signals of their preamplifiers. although several CanBerra amplifiers are suitable, the best resolution for analog MCa systems 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 reset preamplifiers. any of the CanBerra Digital Signal Processing MCas or components can be used with these detectors and provide even better throughput and resolution performance.

MULTIPLE INPUT SYSTEMSCanBerra offers two solutions for multiple input counting systems which process the amplified signals from a number of detectors. a multiple input scenario would typically be considered six or more detector inputs – or the point at which multiple independent MCa systems become cost prohibitive for a given counting application.

The Multiport II (Figure 1.18) is the first solution and, also, the more robust of the two. It offers the capability for up to six totally independent MCas and aDCs housed in one double-wide nIM. Because the MCas and aDCs are separate from each other, any combination of detectors and channel number settings may be used for each input.

8701ADC

Genie 2000 software

HPGeDetector

2002Preamp

PreampPower

INHV

LynxH.V. Inh.

Ethernet

Figure 1.20 HPGe Detector with DSa-2000 Digital Signal Processor (DSP)

Figure 1.19 HPGe Detector with USB-connected Digital Signal Processors

8701ADC

Genie 2000 software

HPGeDetector

2002Preamp

LynxDSA-1000

InSpector 2000

USB

PreampPower

INHV

H.V. Inh.

The second solution employs a Model 8224 Multiplexer (or Mixer/router) to route the signals from multiple detectors to a single aDC for digitizing and on to a 556B MCa for processing as shown in Figure 1.21. Since this configuration shares the MCa and aDC among the detectors, it has a lower cost per input than the Multiport II – particularly for large numbers of detectors. However, the Multiplexer configuration has a major drawback due to the single aDC; the count rate of the individual detectors must be relatively low to avoid excessive signal pileup. additionally, a Multiplexer must allocate the memory of the MCa to its various inputs (same amount for each input), which decreases the number of channels available for each individual detector. Within these constraints, Multiplexers 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. Low-level gamma counting with naI detectors, which typically don’t need more than 1024 channels, is another application that can make use of a Multiplexer. an example configuration is depicted in Figure 1.21.

It should be noted that the Multiport II and the 8224 Multiplexer do not include spectroscopy amplifiers or detector bias supplies. These components must be supplied by other parts of the signal chain. also, these two solutions do not include the benefits of Digital Signal Processing.

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 COUNTINGLarge 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 compute 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:

a. The detector should have an adequate diameter. This assures that the efficiency at medium and low energies will be high relative to the efficiency at 1.33 MeV, where it is bought and paid for.

b. The detector-to-end-cap distance should be minimal – five millimeters or less. The inverse square law is real and will affect sensitivity.

c. The detector should be of closed end coaxial geometry, to assure that the entire front face is active.

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,

improvements in resolution will not improve sensitivity as dramatically as increased efficiency.

8224

Ethernet

3102D

2022

PreampPower

802-3x3 2007P

8224 Mux and 556B MCAwith Genie 2000 software

8807

77 (

554)

RP

I

556B

8701

Figure 1.21 Multiple Input naI Detector System

Sensitivity = 1

Bkg√

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.

GERMANIUM DETECTORS WITH INERT SHIELDSThere 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 1.22 and 1.23.

Figure 1.22 Detector located in center of chamberwithout requirement for extended end-cap

Figure 1.23 Model 747 Lead Shield

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

Shield Specs: Inside DimensionsWall ThicknessMaterial

28 cm dia. x 40.5 cm high10 cmLow Background Lead

HPGe Specs: relative efficiencyresolution

12%1.95 keV FWHM at 1.33 MeV0.90 keV FWHM at 1.22 keV

BackgroundCount:

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

Sensitivity: 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 ENERGYin keV

SENSITIVITYin pC

57Co 139Ce 137Cs 60Co

122 165 662 133

2 3 6 10

LOW BACKGROUND CRYOSTATSThe 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.

HPGe COMPTON SUPPRESSION SPECTROMETERWhen 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 contains only 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 1.24 illustrates a typical example of a 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.

Figure 1.24 Compton Suppression System

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 1.24 yields equivalent results.2 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 1.25. 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 SYSTEMSHigh 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.

2. Compton Suppression Made easy, application note

Figure 1.25 Ge Spectra with Compton Suppression

THE DETECTORFor 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/ns. 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 lesser efficiency. The detector in the latter case will ‘see’ fewer events, and furthermore will have a lower charge collection time.

THE PREAMPLIFIERMost 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.

Table 1.5Major System Components and their Throughput Limitations

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: 1) lower preamplifier noise contributions, 2) inherently better resolution and reduced spectrum broadening vs. count rate, 3) elimination of the need for pole/zero cancellation, and 4) elimination of ‘lock-up’ due to saturation. Figure 1.26 illustrates the throughout performance of the two preamplifier styles.

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.

DIGITAL SIGNAL PROCESSORas we described in an earlier section, Digital Signal Processors (DSP) have come to replace the analog shaping amplifier and aDC in most high performance gamma spectroscopy systems. It is in applications involving high count rate performance where the advantages of DSP become most pronounced.

In gamma spectroscopy systems, the DSP replaces the functionality of both the shaping amplifier and the aDC. The DSP first filters the signal for optimum signal to noise ratio and to provide gain. It then detects the peak amplitude of the filtered pulse to calculate the memory address of the MCa channel into which the event is to be stored.

In the DSP, the analog signal from the preamplifier is first differentiated in the analog domain to provide a rapid return to baseline. This is depicted in Figure 1.27. The resulting time varying voltage signal is sampled by a high speed sampling analog to digital converter. This results in a digitized profile of the differentiated preamplifier signal represented in internal memory of the DSP. From this point on, the signal is processed in the digital domain by the DSP – essentially a high speed digital computer executing calculations as opposed to analog circuits manipulating time varying voltage signals.

Figure 1.26 Throughput vs. Count rate: Throughput Optimization

Figure 1.27 Typical amplifier Pulses

Figure 1.28 Trapezoidal Pulse Waveform as processed in DSP

Processing the signals digitally allows more sophisticated filtering functions to be applied to the signal. It also allows greater flexibility to the user in terms of adjusting filtering parameters – more possible settings are available because they are handled as digital commands, not the selection of discrete analog components. Finally, the use of high speed digital electronics allows the signals to be processed more rapidly, thus contributing further to the count rate performance of the system.

CanBerra’s DSP products deploy a trapezoidal filtering algorithm as shown in Figure 1.28. Two parameters are available for user adjustment – the rise/fall time of the trapezoid (hereafter referred to as rise time) and the flat top time.

adjusting the rise time changes the filter characteristics to optimize for noise characteristics. The larger the rise time, the better the signal to noise ratio. Shorter rise times will adversely affect signal to noise ratio and degrade the resolution of the system. Flat top adjustments are made to accommodate the variations in pulse rise time which in turn is proportional to the charge collection time in the detector. Larger detectors tend to have a larger number of long rise time (large charger collection time) events, thus requiring a longer flat top time. Failure to set the DSP rise time long enough to accommodate the longest charge collection time events results in degraded resolution, an effect known as ballistic deficit. note that for some types of smaller detectors, the flat top time can be set near or very close to zero, resulting in a triangular shape.

Figure 1.29 a comparison of the system throughput as a function of input count rate for a DSP and an analog system optimized for

high throughput for a small detector (11%)

Figure 1.30 a comparison of the system resolution as a function of input count rate for a DSP and an analog system optimized

for maximum throughput for a small detector (11%)

Figure 1.31 a comparison of the system throughput as a function of input count rate for an analog system optimized for maximum

throughput with a DSP system set for a similar throughput

These two parameters together control the total event processing time. The total processing time for an event processed with the DSP trapezoidal algorithm is defined by the equation:

Tp = (2Tr) + Tflat top

We see that the settings for both parameters effect the total processing time, which in turn effects the count rate throughput of the system. as we noted earlier, setting either parameter too fast can result in lost resolution. Increasing the settings improve resolution, but lengthen processing time and sacrifice throughput. a tradeoff exists (as it did in analog systems) between count rate throughput and resolution. Higher throughput can be attained with a loss of resolution and better resolution can be attained at a loss of throughput – up to the limits imposed by the performance of the detector and preamplifier components.

These tradeoffs also existed in traditional analog systems, but the tradeoffs can now be made at a higher level – the DSP provides both improved throughput and improved resolution as compared to analog. This is due to a number of factors. First, the trapezoidal algorithm is simply more efficient and can process the signals more accurately and rapidly than analog electronics.

Secondly, the user has much more flexibility to vary the components of the processing time. In analog systems, the processing was controlled by a single parameter – the shaping time. now with DSP, two parameters are available – one to accommodate noise level and one to accommodate detector pulse rise time. By adjusting these two separately, optimum settings can more readily be attained resulting, generally, in shorter total processing time to reach the same resolution result. additionally, the analog amplifiers typically were limited to six or fewer shaping time selections. If, say, 2 µs shaping was too short, the next available selection was usually 4 µs – twice the processing time. With the CanBerra DSP products, the user can typically select from 35 to 40 rise times and 21 flat top times. again, this greater granularity of adjustment makes it possible to more closely optimize the performance.

note that the CanBerra DSP products also implement Pile Up rejection/Live Time Correction (PUr/LTC). earlier products implemented this feature with analog circuitry, but in the DSP this is incorporated into the digital domain functions. Pulse pileup occurs when a new pulse from the preamplifier reaches the input stages of the DSP before the previous pulse is fully processed. In such cases, the PUr/LTC function a) inhibits the processing of any invalid, composite pulses and b) turns off the live time clock during the time pulse processing is gated off. In this manner, piled up events – which would serve only to distort the spectrum – are rejected before storage by the MCa and the actual live counting time of the MCa remains correct.

The improved performance of the DSP as compared to analog systems is shown in Figures 1.29 to 1.34. Figures 1.29 and 1.30 show real performance data collected with a DSP and an analog gated integrator and fast aDC (the fastest available using analog technology). For this experiment, a Model 2060 DSP was set for rise time of 0.72 µs and flat top time of 0.68 µs. The analog gated integrator amplifier (Model 2024) was set for shaping of 0.25 µs and paired with a 800 ns Fixed Dead Time aDC. These settings were chosen for optimal throughput with a relatively small (≈11% efficient) germanium detector.

as we can see from Figure 1.29, the DSP based system provides higher throughput by approximately 50%. Figure 1.30 shows the resolution comparison for the same experiment and demonstrates that the DSP also provides significantly better resolution once the input count rate exceeds approximately 150 kcps. note that the shape of the resolution curve in Figure 1.30 is also much flatter, indicating that widely varying count rates can be accommodated at a relatively constant resolution.

note that with these settings chosen for highest throughput, the resolution performance at lower count rates is actually slightly worse with the DSP. However, in an application involving those count rates, it is unlikely those settings would be used. Figures 1.31 and 1.32 show the same analog data compared to the DSP system with the rise time extended to 1.24 µs. This reduces the throughput of the DSP system although it is still superior to that of the analog. Further, we see now that with these settings, the resolution of the DSP is superior to the analog across the full range of incoming count rates.

Figures 1.33 and 1.34 compare a Model 2060 DSP to a Gaussian analog system consisting of a Model 2025 amplifier and Model 8715 aDC. In this case, the settings of both systems were chosen to provide optimal resolution under the high incoming count rates. analog systems were set for 2 µs and 4 µs Gaussian shaping times while the DSP settings were 5.6 µs rise time and 0.8 µs flat top. Figure 1.33 shows that, with these settings, the throughput of the DSP system is approximately equal to that of the 2 µs Gaussian system. Yet Figure 1.34 shows the resolution of the DSP system is superior to the 4 µs Gaussian system. again, the DSP allows the spectroscopist to achieve a significantly better tradeoff between throughput and resolution.

LOSS FREE COUNTING APPLICATIONSThe 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 Loss Free Counting (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 µs. Loss free counting requires that the MCa support “add-n” or multiple “add-one” data transfer; consult the factory for details.

Figure 1.32 a comparison of the resolution between an analog system optimized for maximum throughput and a DSP system

set for a similar throughput

Figure 1.33 a comparison of the system throughput as a function of input count rate for a DSP and two

analog systems optimized for resolution

Figure 1.34 a comparison of the system resolution as a function of input count rate for a DSP and two

analog systems optimized for resolution

©2010 Canberra Industries, Inc. all rights reserved.

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” feature with the Lynx which allows the collection of both of these spectra so that statistically correct peak filling can occur.

PIPS DETECTORS AND MULTICHANNEL ANALYZERSalpha 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.

Figure 1.35 example Large Scale alpha Spectroscopy System based on the alpha analyst

an example of a single chamber alpha spectroscopy system (that can easily be upgraded) is illustrated in Figure 1.11. note that the Model 7401 alpha Spectrometer is a complete, self-contained, double-width 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 is more cost effective and user efficient to select a system based on the alpha analyst (Figure 1.35). This turn-key system supports multiple detectors in a comprehensive software environment featuring full computer control of all vacuum elements and acquisition electronics.

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