Tsoulfanidis N. Measurement and detection of radiation

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MEASUREMENT

AND

DETECTION

RADIATION

Figure

1.3

typical pulse-type detector

signal.

shown in Fig. 1.3. For others, the signal may be a change in color (emulsions) or

some trace that can be photographed (bubble or spark chambers)

The ideal pulse-type counter should satisfy the following requirements:

Every particle entering the detector should produce a pulse at the exit of the

counter, which is higher than the electronic noiset level of the unit that

accepts it (usually this unit is the preamplifier). In such a case, every particle

entering the detector will be detected, and the detector efficiency, defined as

the ratio of the number of particles detected to the number of particles

entering the counter, will be equal to 100 percent (for more details on

efficiency, see Chap.

8).

2. The duration of the pulse should be short, so that particles coming in one

after the other in quick succession produce separate pulses. The duration of

the pulse is a measure of the dead time of the counter (see Sec. 2.21) and

may result in loss of counts in the case of high counting rates.

3. If the energy of the particle is to be measured, the height of the pulse should

have some known fixed relationship to the energy of the particle. To achieve

this, it is important that the size of the counter is such that the particle

deposits all its energy (or a known fraction) in it.

If two or more particles deposit the same energy in the detector, the

corresponding pulses should have the same height. This requirement is

expressed in terms of the energy resolution of the detector (see Chap.

9).

Good energy resolution is extremely important if the radiation field consists

of particles with different energies and the objective of the measurement is to

identify (resolve) these energies. Figure 1.4 shows an example of good and

bad energy resolution.

There is no detector that satisfies all these requirements. Few detectors

have 100 percent efficiency. In practice, it is not feasible for gamma and neutron

detectors to have all the energy of the particle deposited in the counter. Because

of statistical effects, there is no detector with ideal energy resolution. What

should one do?

'~lectronic noise is any type of interference that tends to "mask" the quantity to be observed.

It is usually 'the result of the thermal motion of charge carriers in the components of the detection

system (cables, resistors, the detector itself, etc.) and manifests itself as a large number of low-level

pulses. Electronic noise should be distinguished from background pulses resulting from radiation

sources that are always present,

e.g., cosmic rays.

INTRODUCTION

RADIATION

MEASUREMENTS

Good

Figure

1.4 Good and bad energy

Energy

resolution.

In practice, the experimenter selects a detector that satisfies as many of

these properties as possible to the highest degree possible and, depending on

the objective of the measurement, applies appropriate corrections to the mea-

sured data.

1.5.3

The

NIM

Concept

Most of the commercially available instruments that are used in radiation

measurements conform to the standards on nuclear instrument modules (NIM)

developed by the U.S. Atomic Energy Commission (now the Nuclear Regulatory

Commission) and now dictated by the Department of ~ner~~.'

The objective of the NIM standard is the design of commercial modules that

are interchangeable physically and electrically. The electrical interchangeability

is confined to the supply of power to the modules and in general does not cover

the design of the internal circuits.

The size of the smallest, called a single-width, NIM is 0.222 m

0.035 m

(8.71 in

1.35 in). Multiple-width NIMs are also made. The standard NIM bin

will accommodate 12 single-width NIMs or any combination of them having the

same total equivalent width. Figure 1.5 is a photograph of the front and back

sides of a commercial standard bin. Figure 1.6 is a photograph of the bin filled

with NIMs of different widths, made by different manufacturers.

1.5.4

The High-Voltage Power Supply

The

high-voltage power supply

(HVPS) provides a positive or negative voltage

necessary for the operation of the detector. Most detectors need positive high

voltage (HV). Typical HVs for common detectors are given in Table 1.2. The

HVPS is constructed in such a way that the HV at the output changes very little

even though the input voltage (110 V, ac) may fluctuate.

10 MEASUREMENT

AND

DETECIlON OF RADIATION

Figure

1.5

Photographs of the

(a)

front

and

(b) back sides of

commercial

NIM

bin (from Canberra

1979-1980 catalog).

INTRODUCTION TO RADIATION MEASUREMENTS 11

Figure

1.6

typical bin filled with a combination of NIMs made by different manufacturers.'

typical commercial

HVPS

is shown in Fig.

1.7.

The front panel has an

indicator light that shows whether the unit is on or off and, if it is on, whether

the output is positive or negative voltage. There are two knobs for voltage

adjustment, one for coarse changes of

500-V

intervals, the other for changes of

0.1

The output is at the rear of the unit.

1.5.5

The Preamplifier

The primary purpose of the preamplifier is to provide an optimized coupling

between the output of the detector and the rest of the counting system. The

preamplifier is also necessary to minimize any sources of noise that may change

the signal.

Table

1.2

High Voltage Needed for Certain Common Detectors

Detector

High

voltage

(V)

Ionization counters

1000

Proportional counters

500

1500

counters

500

1500

Semiconductor detectors

Surface-barrier

100

Lidrifted

100

3000

MEASUREMENT

AND

DETECTION

RADIATION

The signal that comes out of the detector is very weak, in the millivolt (mV)

range (Fig. 1.3). Before it can be recorded, it will have to be amplified by a

factor of a thousand or more. To achieve this, the signal will have to be

transmitted through a cable to the next instrument of the counting system,

which is the amplifier. Transmission of any signal through a cable attenuates it

to a certain extent. If it is weak at the output of the detector, it might be lost in

the electronic noise 'that accompanies the transmission. This is avoided by

placing the preamplifier as close to the detector as possible. The preamplifier

shapes the signal and reduces its attenuation by matching the impedance of the

detector with that of the amplifier. After going through the preamplifier, the

signal may be safely transmitted to the amplifier, which may be located at a

considerable distance away. Although some preamplifiers amplify the signal

slightly, their primary function is that of providing electronic matching between

the output of the detector and the input of the amplifier.

There are many types of commercial preamplifiers, two of which are shown

in Fig. 1.8. In most cases, the HV is fed to the detector through the preamplifier.

1.5.6

The

Amplifier

The main amplification unit is the amplifier. It increases the signal by as many

as 1000 times or more. Modern commercial amplifiers produce a maximum

signal of 10 V, regardless of the input and the amplification. For example,

consider a preamplifier that gives at its output three pulses with heights 50

mV,

100 mV, and 150 mV. Assume that the amplifier is set to 100. At the output of

the unit, the three pulses will be

Note that the third value should be 15

but since the amplifier produces a

maximum signal of 10 V, the three different input pulses will show, erroneously,

as two different pulses at the output. If only the number of particles is

measured, there is no error introduced-but if the energy of the particles is

measured, then the error is very serious. In the example given above, if gammas

of three different energies produce the pulses at the output of the preamplifier,

the pulses at the output of the amplifier will be attributed erroneously to

gammas of two different energies. To avoid such an error, an observer should

follow this rule:

Before any measurement of particle energy, make certain that the highest pulse of the

spectrum to be measured is less than

at the output of the amplifier.

In addition to signal amplification, an equally important function of the

amplifier is to convert the signal at the output of the preamplifier into a form

INTRODUCITON TO RADIATION MEASUREMENTS

Figure

1.8

Two typical commercial preamplifiers:

(a)

type used with a photomultiplier tube (made

by Harshaw), and

(b)

type used with semiconductor detectors (made by Canberra).

MEASUREMENT

AND

DETECTION

RADIATION

suitable for the measurement desired. More details on this subject are given in

Chap. 10. The front panel of a typical commercial amplifier is shown in Fig. 1.7.

Commercial amplifiers have two dials for adjusting the amplification.

Coarse gain: This dial adjusts the amplification in steps. Each step is a

fraction of the maximum amplification. For example, the dial may show the

numbers 1,

8, 16. If the maximum amplification is 100, then the coarse

gain on 16 will give a maximum of 100, the coarse gain on 8 will give 50, etc.

Some amplifiers have the numbers

1, and some newer ones have

1, 10, 100, 1000, etc.

Fine gain: This dial adjusts the amplification continuously within each step of

the coarse gain. The numbers, in most units, go from 0 to 10. The highest

number provides the maximum amplification indicated by the coarse gain. As

an example, consider the maximum amplification to be 100. If the coarse gain

(highest number 16) and the fine gain 5 (highest number lo), the

amplification will be 100

(coarse gain)

(fine gain)

25.

Most commercial amplifiers provide at the output two types of pulses, called

unipolar and bipolar (Fig. 1.9).

1.5.7

The Oscilloscope

The oscilloscope is an instrument that permits the study of rapidly changing

phenomena, such as a sinusoidal voltage or the pulse of a counter. The

phenomenon is observed on a fluorescent screen as shown in Fig. 1.10. The

horizontal axis of the screen measures time. The vertical axis gives volts.

In radiation measurements the oscilloscope is used to check the quality of

the signal as well as the level and type of the electronic noise. It is always a good

practice before any measurement is attempted to examine the signal at the

output of the amplifier. A few examples of good and bad pulses are shown in

Fig. 1.11. In Fig. 1.11,

and

represent good pulses, and Fig.

1.11~

is probably

(a)

(6)

Figure

1.9

The pulse at the output of the amplifier:

(a)

unipolar pulse and

(b)

bipolar pulse.

INTRODUCTION

RADIATION

MEASUREMENTS

Figure

1.10

Two commercial oscilloscopes:

a Tektronix

2212

oscilloscope (Copyright

1994 by

Tektronix, Inc. All rights resewed. Reproduced

permission.); and

Philips PM3394A

autoranging combiscope (Reproduced with permission).

an electrical discharge, not good for counting. Figure

l.lld

is no good either,

because a high-frequency signal is "riding" on the output of the preamplifier. If

the pulse is not good, the observer should not proceed with the measurement

unless the source of noise is identified and eliminated.

Modern oscilloscopes provide analog as well as digital signals.

1.5.8 The Discriminator or Single-Channel Analyzer (SCA)

The

SCA

is used to eliminate the electronic noise and, in general, to reject

unwanted pulses. When a pulse is amplified, the electronic noise that is always

present in a circuit is also amplified. If one attempts to count all the pulses

present, the counting rate may be exceedingly high. But electronic noise is a

nuisance and it should not be counted.