Detection and Measurement of Nuclear Radiation (1962) / Chapter Skim
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Auxiliary Electronic Instrumentation
Auxiliary Electronic Instrumentation
Pages 88-110
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From page 88... ...
The known half life may be fitted accurately to the points at low rates, for which the corrections are small or negligible, and the decay curve is extrapolated to time zero. The differences between the decay curve and the experimental points may be used to construct a correction curve directly or to compute the resolving time.
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From page 89... ...
Before describing the various components themselves, it may be noted that transistorized electronic devices are now available which are essentially equivalent to equipment using vacuum tubes. Thus, linear amplifiers, scalers, and pulseheight analyzers generally may be obtained in either transistorized or vacuum-tube versions.
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From page 90... ...
Detector Energy Dependent Rise Time G-M 0-10 volts No Slow Proportional 0-100 mv Yes Slow Pulse Ion Chamber 0-3 mv Yes Slow Semiconductor 0-25 mv Yes Fast Scintillation 0-2 volts Yes Fast For a circuit capacitance of 20 pf. that the pulse heights obtained are very small, and so an amplifier is needed between the detector and measuring apparatus.
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From page 91... ...
detector; for example, the rise time of a scintillation detector pulse depends on the decay time of the fluorescent light and the photomultiplier characteristics. The fall time t.
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From page 92... ...
If two time constants in the amplifier are equal to each other and are smaller than the remaining networks, then the undershoot will have the largest amplitude and the shortest duration. This pulse shaping is known as double differentiation or double clipping; a typical waveform is shown in Fig.
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From page 93... ...
A disadvantage is that the delay-line circuits often used give rise to a small undershoot, which causes a base line shift at high counting rates. The best pulse shape for most work is obtained by double clipping with delay lines.
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From page 94... ...
Such harsh treatment can lead to blocking, or a temporary amplifier paralysis following an overload pulse. Naturally, if high counting rates are to be tolerated, the amplifier should recover quickly after an overload.
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From page 95... ...
Presently available transistors cannot equal the low noise performance of vacuum tubes at low input capacitances. With input capacitances of 15 to 20 pf, transistor amplifiers exhibit 3 to 10 times more noise than the best vacuum-tube amplifiers.
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From page 96... ...
Since the output is essentially proportional to q alone, the pulse height does not vary with input capacitance, as has already been mentioned in connection with semiconductor radiation detectors (Section IV.3.A.)
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From page 97... ...
Electromechanical registers can only accommodate counting rates up to 60 per sec, while many experiments demand recording data at rates of 30,000 counts/sec or more. This is accomplished by dividing the number of incoming pulses by a known factor (the scaling factor)
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From page 98... ...
Improvements in electronic components and circuitry during the past few years have made it possible to construct decimal scalers which are as reliable as the binary ones. A functional diagram for a transistorized decimal scaling stage usable to a pulse rate of 1 Me is shown in Fig.
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From page 99... ...
BINARY SCALER BQ RESET FEEDBACK u , S> rv -- - -- 'iS TVi i 2 4 ( 3i ;, i DECIMAL SCALER Fig.
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From page 100... ...
Each pulse transfers a known charge to the tank capacitor; the steady-state voltage developed across the tank capacitor is reached when the rate of charge loss through the shunt resistor equals the rate of charge input from the pulses. A good quality vacuum-tube voltmeter is used to indicate the voltage across the RC tank circuit.
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From page 101... ...
The E dial in the case shown is set at 200 dial divisions; pulse "1" does not have sufficient amplitude to affect either PHS. Pulse "2" falls within the AE window, which causes the lower PHS to trigger; as there is no accompanying pulse from the upper PHS, an output pulse is recorded.
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From page 102... ...
levels are progressively increased. Anticoincidence circuitry is provided, so that, in effect, the array consists of a series of single-channel analyzers, 102
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From page 103... ...
The 20-channel analyzer just mentioned costs about $350/channel; hence, a stacked-discriminator type of analyzer is too expensive to build in large configurations. By making use of techniques developed for digital computers, it is possible to construct multichannel analyzers having hundreds of channels SIGNAL INPUT OUTPUT .
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From page 104... ...
The dependence of the rather long "dead time" on channel number shows that the average dead time is a function of the spectrum under measurement. However, it can be shown that the spectrum shape is undistorted under this condition, and so it is only necessary to correct for the dead time to obtain accurate counting rates.
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From page 105... ...
7. Coincidence Measurements In many nuclear counting problems, it is necessary to decide whether two events are time-correlated.
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From page 106... ...
Short resolving times are required wherever high counting rates are involved, because the random nature of radioactive decay leads to a chance that two uncorrelated pulses will happen to occur within the coincidence resolving time. The random coincidence rate N is given by Nr = Zr N,N2 , (16)
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From page 107... ...
The output from the usual trigger circuits will exhibit such a walk, because low-amplitude pulses will trigger near their peaks, while high-amplitude pulses will trigger proportionately nearer the base line. The output pulses will be distributed through a time range about equal to the amplifier rise time (~ 0.2 \isec for Nal(Tl)
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From page 108... ...
For use with a multichannel analyzer, the connection to single-channel analyzer "1" is broken at "X," and the connection shown as a dotted line is used. Often, it is desirable to include a multichannel analyzer in a coincidence arrangement in order to measure the spectrum at one detector in coincidence with a selected energy from another detector.
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From page 109... ...
Coincidence techniques are well suited to the measurement of very short halflives. If delay lines are inserted between the timing circuit and fast-coincidence input, first in one channel and then the other, a delay curve can be obtained, which is just the coincidence counting rate as a function of added delay.
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From page 110... ...
(19) is useful in many situations, two additional corrections are needed: one to correct for the fact that only a fraction of the counts in the window may be due to y2 ; it is also very important to remove the coincidence contribution from higher-energy gamma rays which give Compton-electron events falling in the single-channel window.
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