keV (see figure 3.26).

Finally, the timing characteristics are noted. Best timing

resolution (~ 10 ns FWHM) is achieved with thin planar detectors for

which the charge-collection field is most uniform. Large-volume

coaxial detectors yield a timing resolution of about 20 ns; the

electric field is non-uniform, decreasing towards the edge of the

crystal. The method of deriving the timing output is important in

this context and is discussed further in section 2.5.2. Here it is

noted that since the rise time of the current pulse is dependent on the energy deposited in the crystal (time walk), even with constant- fraction discrimination there is some tendency for the zero of the time output to shift with energy. However, the use of event-by-event recording reduces the problems associated with time walk.

2.4.2 Other Detectors — Nal (Tl) and NE213

Thallium-activated sodium iodide, Nal(Tl), scintillation counters were used to record y-ray events as part of a multiplicity filter (see

section 2.5.3). These detectors have superior peak-efficiency to

equivalent size Ge(Li) detectors but inferior energy resolution. In

the present experiments the Nal(Tl) detectors were used only to register the presence of events above some threshold energy, so that their particular characteristics are not of great interest. A photo­ multiplier is used to amplify the scintillation pulse and best timing

resolution is obtained by taking the output from the anode. The

timing resolution is anyway superior to that of Ge(Li) detectors, and was not a limiting factor in the present experiments.

An NE213 organic scintillator was used for neutron detection. Such scintillators are sensitive to both y-rays, for which the largely Compton-scattered electrons leave relatively low-density ionisation tracks, and neutrons, for which the knock-on protons have high-density

ionisation tracks. The decay of the scintillation pulse contains both

fast and slow components, with relative intensities depending upon the

ionisation density: for high ionisation densities, corresponding to

decay time of the derived current pulse is used to select the neutron events and reject the y-ray events, as discussed further in section

2.5.2. Higher-energy neutrons result in lower-density ionisation

tracks and become indistinguishable from y-rays, but for the neutron energies encountered (about 3 MeV) the separation is adequate (see

figure 2.4).

2.5 DETECTOR SYSTEMS, ELECTRONICS

AND DATA STORAGE

2.5.1 Gamma-ray Singles Measurements

High-resolution Tennelec 205A linear amplifiers with 3 ]is shaping time were used to amplify the output from the Ge(Li) detector pre­

amplifiers. Counting rates were kept at about 8 kHz in order to main­

tain good resolution and reduce pile-up (about 3% at 10 kHz). System

dead-time was measured by counting the number of events from each linear amplifier and the corresponding number stored in the computer

(HP2100A-5406B analyser). Dead-times of about 30% were recorded for a

typical two-detector arrangement. In order to minimise the relative

corrections between different detector configurations, counting rates wore kept approximately constant (this was also important because

amplifier dead-time was not monitored). Thus in a typical y-ray

angular distribution measurement, with one monitor detector being fixed at 90° and one movable detector being placed at various angles,

relative corrections of only about 5% were necessary. Due to the

computer interface not accepting coincident events when in singles mode, the coincidences in the monitor detector were inhibited prior to presentation at the interface.

The recorded spectra were written on magnetic tape for subsequent off-line analysis.

2.5.2 Neutron-gamma and Gamma-gamma Coincidence Measurements

Standard fast/slow electronics was used for all coincidence

detectors was obtained by amplifying (timing filter amplifier — Ortec 454) the timing output of the detector pre-amplifier, followed by

constant-fraction discrimination (Ortec 473). The small-volume planar

detectors have high-gain pre-amplifiers, and an "extrapolated zero strobe" (Canberra 1426), which also compensates for time walk, was used. Constant-fraction discriminators were used directly with the

scintillator timing outputs. The difference between timing signals,

with their delays as appropriate, was converted to a linear amplitude

output (TAC — Canberra 1443) . The above were carried out in the

proximity of the detectors, while the stow electronics was situated

close to the computer. Here coincidences were established between the

timing and energy signals (over the full TAC range of typically 1 fis) and this logic signal (from Canberra 1446) gated the three linear signals (Canberra 1454) from the two detectors and the TAC, before

being converted to digital format (Canberra &ö6o); these three

signals were written event-by-event in three-parameter format (dx, d? ,

t12) on magnetic tape for subsequent off-line analysis.

Neutron identification was achieved using a pulse-shape analyser (Ortec 458) which gives an output whose amplitude is proportional to

the 90% to 10% decay time of the input signal. Thus the neutrons,

for which the fast decay component is quenched (see section 2.4.2), yield larger amplitudes, as is illustrated in figure 2.4. Lead shielding was used to attenuate the y-rays.

Examples of time spectra are shown in figure 2.5 for two

transitions in 175W (see also figure 3.5). In the following

discussions, a coincidence will be used to refer to two events within

the TAC time range (about 1 ys) , while a prompt coincidence refers to

two events within the resolving time (about 15 ns) ,- delayed

coincidences occur more than about 15 ns after the initial event.

2.5.3 The Use of a Multiplicity Filter

The technique is similar to that described by Hagemann et at.

[Ha 75]. Six 5 cm x 5 cm Nal(Tl) detectors were placed symmetrically