An overview of a typical data acquisition setup

COMMON COMPONENTS

PREAMPLIFIERS

General

The fundamental output of all pulse type radiation detectors is a burst of charge Q liberated by the incidentradiation. For most detectors the charge is so small that it is impractical to deal with the signal pulses without an

intermediate amplication step. The rst element in a signal-processing chain is therefore often a preamplier as

an interface between the detector and the pulse-processing and analysis electronics that follow.

The preamplier is usually located as close as possible to the detector. From a signal-to-noise standpoint, it

is always preferable to minimize the capacitive loading on the detector, and therefore long interconnecting cables

between the detector and preamplier should be avoided if possible. One function of the preamplier is to terminate

the capacitance quickly and therefore to maximize the signal-to-noise ratio. It also serves as an impedance matcher,

presenting a high impedance to the detector to minimize loading, while providing a low impedance output to drive

succeeding components.

The preamplier conventionally provides no pulse shaping, and its output is a linear tail pulse. The rise time

of the output pulse is kept as short as possible, consistent with the charge collection time in the detector itself.

The decay time of the pulse is made quite large (typically 50 or 100 s ) so that full collection of the charge fromdetectors with widely diering collection times can occur before signicant decay of the pulse sets in.

Voltage- and charge- sensitive Congurations

Figura 1: Schematic diagram of a simplied voltage-sensitive preamplier

conguration. R2 is the feedback resistance.

Figura 2: Simplied diagram of a charge-sensitive preamplier congu-

ration. If the conditions indicated are met, the output pulse amplitude

becomes independent of the input capacitance Ci. The time constant givenby the product RfCf determines the decay rate of the tail of the outputpulse.

Preampliers can be of either the

voltage-sensitive or charge-sensitive

type. Historically, the voltage-

sensitive type is the more conventio-

nal in many electronic applications

and consists simply of a conguration

that provides an output pulse who-

se amplitude is proportional to the

amplitude of the voltage pulse sup-

plied to its input terminals. A sche-

matic diagram of a voltage-sensitive

conguration is shown in Fig. 1. If

the time constant of the input circuit

(the parallel combination of the input

capacitance and resistance) is large

compared with the charge collection

time, then the input pulse will have

an amplitude equal to

V =Q

C(1)

where C is the input capacitance. For

most detectors, the input capacitance

is xed so that the output pulse pro-

duced by a voltage-sensitive pream-

plier is proportional to the charge Q

liberated by the incident radiation. If

the input capacitance were to change

however, this desirable proportionali-

ty would no longer hold. In semicon-

ductor diode detectors, for example,

the detector capacitance may change

with operating parameters. In these

situations, a voltage-sensitive preamplier is undesirable because the proportionality between Vmax and Q is lost.The elements of a charge-sensitive conguration that can remedy this situation is shown in Fig. 2. For this

circuit, the output voltage is proportional to the total integrated charge in the pulse provided to the input terminals,

as long as the duration of the input pulse is short compared with the time constant RfCf . Changes in the inputcapacitance no longer have an appreciable eect on the output voltage. Although originally developed for use with

semiconductor diode detectors, this charge-sensitive conguration has proved its superiority in a number of other

applications, so that preampliers used with other detectors in which the capacitance does not necessarily change

are also often of the charge-sensitive design.

Noise Characteristics

Probably the most important specication for a preamplier is its noise gure. This specication is normally

quoted as the FWHM of the response function of the system due only to the preamplier noise. The gure is

normally given as the equivalent energy spread in the type of detector for which the preamplier is designed. The

noise gure is a strong function of the capacitance with which the preamp input is loaded. For example good quality

preamplier used with silicon diode detectors may have a noise gure of 2 keV with zero input capacitance, but this

gure may double if the input is loaded with 100 pF. The input capacitance arises from both the inherent detector

capacitance and from the connecting cable between the detector and preamplier. It is therefore important to keep

the interconnecting cable as short as possible and to choose a detector whose inherent capacitance is no larger than

necessary. The rise time for charge-sensible preampliers also normally increases with input capacitance.

For a wide assortment of applications, the noise level of commercially available preampliers is suciently low

so that their contribution to the FWHM of the system response is small compared with the inherent contributions

of the detector itself.

Detector Bias Voltage

Figura 3: two congurations for supplying detector bias through a

preamplier. (a ) an ac-coupled arrangement, in which a coupling

capacitor is provided between the detector and preamplier circuits.

This allows interchanging dierent values of RL without aecting thepreamplier input. (b) A de-coupled conguration that eliminates

the coupling capacitor and generally leads to better noise performan-

ce for critical applications. Now the detector must be isolated from

ground, and changing the bias resistor RL may aect the input stagecharacteristics.

Another function normally carried out by

the preamplier is to provide a means for

supplying bias voltage to the detector. Ar-

rangements can always be made to supply

voltage to the detector independent of the

preamplier, but it is usually convenient

to do so through the preamplier. In Fig.

3. two congurations are shown in whi-

ch the bias voltage is supplied through a

load resistance R

L

A single cable between

the preamplier and detector provides bo-

th the voltage to the detector and the si-

gnal pulse to the input of the preamplier.

The load resistance, together with the in-

put capacitance, determines the time con-

stant across which the detector current is

collected. From the standpoint of mini-

mum noise, R

L

should be as large as pos-

sible. Practical limitations always dictate

that its value be no more than a few thou-

sand megohms due to the fact that any dc

signal or leakage current drawn by the de-

tector must also ow through this resistor.

Leakage currents are especially troubleso-

me for semiconductor diode detectors and,

in those cases in which R

L

is large, can lead

to a substantial dc voltage drop across R

L

.

In that event, the voltage actually applied

to the detector is less than the supply vol-

tage, and experimenters must be aware of

the magnitude of the leakage current so

that they can compensate for the volta-

ge drop across R

L

by raising the supply

voltage.

The best noise performance in present

designs results from the use of a eld eect

transistor (FET) as the input stage. FETs are notoriously sensitive to overvoltage transients and can easily be

damaged by the transients that can be generated by switching a detector bias supply in coarse steps or abruptly

disconnecting or turning o the voltage. As a result, many commercial preampliers are provided with overvoltage

protection circuits. For the ultimate in low noise performance, however, it is often necessary to switch out the

protection circuits, and in such circumstances the bias supply must be changed only gradually and in a continuous

fashion.

Test Pulse Input

Most preampliers are also provided with an input labeled test pulse, which is intended to receive the output

of a pulse generator for system test purposes. Fig. 4 shows one means of applying this test pulse to the input

stage of a charge-sensitive conguration. If the pulser is suciently stable, its output should be resolved into a

single channel by a pulse height analyzer connected to the system output. Any broadening of this response into

more than one channel can be attributed to the inherent electronic noise of the measurement system. It is often

interesting to measure this inherent electronic noise width because any measured detector response can never be

better resolved than this electronic limit. It is important to make this measurement with the detector attached to

the input of the preamplier because its capacitance is often a signicant factor in the overall noise characteristics

of the preamplier. The test pulse input provides a convenient means of carrying out this noise determination

as well as simply checking the overall function of the signal-processing system prior to a measurement. In more

sophisticated systems, a test pulse input may also be required during the course of a measurement for purposes of

dead time determination or gain stabilization

Figura 4: A charge sensitive preamplier that has been provided with a test pulse input. If a step voltage pulse

of amplitude Vi is applied to this input, a charge equal to ViCi is supplied to the preamplier input stage. Rt is asmall-value termination resistance.

DETECTOR BIAS AND HIGH-VOLTAGE SUPPLIES

With few exceptions, virtually all radiation detectors require the application of an external high voltage for their

proper operation. This voltage is conventionally called detector bias, and high-voltage supplies used for this purpose

are often called detector bias supplies.

Some characteristics of detector bias supplies which can be important in specic applications are the following:

1. The maximum (and minimum) voltage level and its polarity.

2. The maximum current available from the supply.

3. The degree of regulation against long-term drifts due to changes in temperature or power line voltage.

4. The degree of ltering provided to eliminate ripple at power line frequency or other low-frequency noise.

The sophistication required of the bias supply varies greatly with the detector type. For detectors that draw very little

current (such as an ion chamber) the bias supply can be as simple as a dry cell battery. On the other hand, supplies that

must simultaneously provide high voltage and relatively high current involve a substantial amount of design engineering and

can be among the heaviest and bulkiest of the equipment normally found in a nuclear instrumentation system.

However, the degree of regulation and ltering is again important because any high-voltage uctuations appear superim-

posed on the signal. Semiconductor diode detectors draw relatively little current and the voltage demands seldom exceed

1000 V.

The voltage level on most bias supplies is adjustable either through switching in steps 1 or by means of a continuously

adjustable helipot. When used with preampliers with FET ; input stages, continuous adjustment avoids switching transients

that may be potentially damaging to the FET. Some designs provide alternate protection by limiting the rate of change of

the voltage between steps.

Figura 5: Elements of typical signal chain for pulse counting

PULSE GENERATORS

An electronic pulse generator is indispensable in the initial setup and calibration of virtually any nuclear instrumen-

tation system. Furthermore, some methods of gain stabilization and dead time determination require the output

of a pulse generator to be mixed with signal pulses during the course of a measurement. Pulsers are therefore a

very common element in most radiation instrumentation systems.

A tail pulse generator with adjustable rise and decay times is probably the most useful of all pulser types. Its

output is conveniently fed to the test pulse input on preampliers or used directly in place of the preamplier

output. If the output amplitude is truly constant, a measurement of the amplitude distribution recorded by the

pulse analysis system determines the electronic noise level present in the system.

Most pulse generators also provide a front panel adjustment for the amplitude of the pulse. In some designs,

often called precision pulsers, this adjustment is accurately controlled by a front panel dial. Such pulsers can then

be used to check the integral linearity of a pulse-handling system simply by recording the output amplitude for

several dierent settings of the input pulse amplitude. The output of most precision pulsers is suciently stable

so that, in the absence of electronic noise, all pulses of constant amplitude would be resolved into a single channel

in a multichannel analysis system.

For normal pulse generators, the interval between pulses is uniform and periodic. However, for determining

some system parameters such as pileup behavior and other time-dependent phenomena, a periodic source does

not adequately represent the random time spacing encountered from actual radiation detector pulses. Therefore,

designs have evolved which provide a source of randomly spaced pulses of constant amplitude. In such random

pulsers, the noise signal from an internal component is often used to trigger randomly the time at which a pulse is

produced at the output.

PULSE HEIGHT ANALYSIS SYSTEMS

General Considerations

Next to the simple counting of pulses, the most common procedure in nuclear measurements involves recording the

amplitude distribution of pulses produced by a radiation detector. Most often the object is to deduce properties of

the incident radiation from the position of peaks in the recorded spectrum, although other aspects of the spectrum

may be of interest in dierent situations. The performance required of the instrument system used to record the

pulse height spectrum is largely dependent on the inherent energy resolution of the detector. If the detector energy

resolution is relatively poor, the requirements of the recording system are undemanding and easy to meet. On

the other hand, detectors with good energy resolution require careful attention to the pulse processing system to

assure that additional degradation of the resolution is minimized.

A simple pulse height analysis system is shown in Fig. 6The key element in this signal chain is the linear

amplier, which shapes the pulses from the preamplier and provides enough amplication to match the input span

for which the multichannel analyzer has been designed. It is the shaping function of the linear amplier that often

dominates the performance of the pulse-processing system. For high-resolution detectors, however, consideration

must be given to the eect of various shaping methods on the pulse properties with regard to signal-to-noise and

pileup.

Figura 6: Scheme of a rudimentary system for non critical applications .

The strategies employed in choosing parameters for the pulse-processing system change drastically with the

expected counting rate. If the counting rate is low (say 100 counts/s or less), the system can be optimized with

regard to processing each individual pulse without much concern for interfering eects between pulses.

1

1

A quick estimate of the importance of rate-related eects can be made by calculating the duty cycle obtained by multiplying the

eective width of the shaped pulse by the rate. If this product is less than about 103, these eects should be minimal and can oftenbe neglected. A duty cycle of 102 is a moderate rate, whereas in high-rate situations it may approach 101. For a typical pulse widthof 5 p.s, the corresponding rates are 200, 2000, and 20,000 per second.

Figura 7: Amplier module