Page 242 - Radiochemistry and nuclear chemistry

226 Radiochemistry amt Nuclear Chemistry

8.7.5. Multichannel analyzers

Multichannel analyzers (MCA) consisting of an analog to digital converter (ADC),
controller and storage unit which may have 16 000 channels or more (i.e. the energy scale
is split up into that number of steps). In this case the pulses are sorted immediately into the
various channels which record the counts as they occur rather than scan over an energy
range in steps. In many designs the MCA is interfaced to a computer to provide display,
recording and analysis of the energy spectrum. MCAs are also made as circuit boards that
fit inside a personal computer and have suitable software by which the PC can emulate a
stand-alone dedicated MCA. The ADC unit is normally based on one of two principles.
In a Wilkinson ADC, the beginning of an input pulse starts a pulser (the clock) and a
ramp voltage. When the input signal culminates its amplitude is "frozen" by a sample-and-
hold gate. The pulser runs until the ramp voltage crosses the frozen input signal level. By
accumulating the pulser signals in a counter the f'mal count is proportional to the amplitude
of the input pulse. This count can then be used as a digital address, channel, telling the
attached digital circuitry where to increment the stored counts. The dead time of this ADC
type is roughly proportional to the amplitude of the input pulse and to the highest address
permitted. A typical conversion time with a 100 MHz clock may be 1 + 0.01 n #s, for
channel n. Wilkinson ADCs are mostly used in MCAs with a relatively small number of
channels.
The successive approximation ADC locks the maximum amplitude in a sample-and-hold
circuit and uses an ultra fast digital voltmeter of successive approximation type to measure
the amplitude. A typical conversion time is ~ 25 tts at 4000 channels full scale. The
voltmeter reading is then used as address in the same way as for the Wilkinson ADC. This
type of ADC has usually a fixed dead time, independent of the pulse amplitude and only
moderately dependent on the maximum address (each doubling of the number of channels
adds the same increment to the conversion time). Successive approximation ADCs are
preferred for MCAs with very many channels as they then, on the average, become faster
than Wilkinson ADCs. On the other hand the linearity of a Wilkinson ADC is usually
slightly better than for a successive approximation ADC.
MCAs are probably the most versatile instruments for nuclear particle detection because
of their usefulness both for qualitative identification and quantitative determination of
radioactive nuclei. Practically all a- and ?-spectra reproduced in this book have been
obtained through this technique.
In case correlated events registered by several ADCs are measured the addresses from the
ADCs and time is usually recorded digitally on a magnetic storage medium in real time.
The desired information, e.g. ?-? coincidences, is then obtained by reading and sorting
the stored data after the experiment.

8.7.6. ?-spectrometry

As we learned in w the capture of a ?-ray in an absorber such as a NaI(TI)- or Ge-
crystal occur by any of three processes - photoelectric effect, Compton effect, and pair
production. In energy analysis of ?-rays it is desirable to capture the total energy and to
minimize the loss of energy by escape of the scattered ?-rays from Compton interaction.