Service manual

10
The slow rise time reject mode is particularly useful
with HPGe detectors operated over a wide dynamic improving timing spectroscopy techniques. Please
range of energies. Operation in the SRT mode will contact your local ORTEC representative
have minimum effect on the FWHM resolution but concerning any special requirements.
can dramatically improve the FWTM and
FW(1/100)M values. The disadvantage of the SRT
mode is that the effective relative efficiency of the
HPGe detector for timing is reduced since some
events of valid energy are removed from the timing
spectra.
The timing resolution obtainable with HPGe
detectors also depends on many variables. The
active volume of the HPGe detector is a major
factor in determining timing resolution.
The charge collection time increases as the
detector volume increases. Charge collection time
variations result in variations in the shape and rise
time of the preamplifier output signal. For this
reason, Amplitude and Rise Time Compensated
(ARC) timing is used in HPGe detector applications.
The principal difference between ARC timing and
CF timing is the selection of the CF shaping delay.
In ARC timing, the CF shaping delay selected is
less than the minimum rise time of the 579 output
signal.
The optimum CF shaping delay for an HPGe
detector is usually obtained experimentally. An
ORTEC 425A Nanosecond Delay unit can be used
to vary the shaping delay of the 584 while taking a
series of timing spectra. A typical value for the
shaping delay for a 15% HPGe detector is in the
range of 20 ns to 30 ns.
Figure 6.3. shows the timing resolution FWHM for
14 HPGe detectors ranging in size from 10% to
35% relative efficiency. Note that individual
detectors can deviate significantly from the mean
value of timing resolution.
6.4. TIMING WITH OTHER DETECTORS
The 584 can be used to provide timing information
for other detectors such as ORTEC Surface Barrier
Detectors and Low-Energy Photon Detectors. When
input signals are low level and have very fast rise
times, an ORTEC 9301 or 9305 Fast Preamplifier,
or an ORTEC 574 or 535 Quad Fast Amplifier are
recommended as accessory modules.
ORTEC conducts a continuing program aimed at
Fig. 6.3. Timing Resolution FWHM for 14 Detector
Systems as a Function of Efficiency for the
Energy Range 511 ± 50 keV for Na.
‘ ’
6.5. REFERENCES
Use the following references for further information
on typical timing coincidence measurement
systems in which the ORTEC 584 Constant-
Fraction Discriminator can be used.
1. "Principles and Applications of Timing Spectros-
copy," Application Note AN 42, ORTEC, Oak Ridge,
TN (1982).
2. T. J. Paulus, T. W. Raudorf, B. Coyne, and R. C.
Trammell, "Comparative Timing Performance of
Large Volume HPGe Germanium Detectors," IEEE
Trans. Nucl. Sci.,
NS-28,
No. 1, pp. 544-548 (1981).
3. M. 0. Bedwell and T. J. Paulus, "A Constant-
Fraction Differential Discriminator for Use in Fast
Timing Coincidence Systems," IEEE Trans. Nucl.
Sci.,
NS-26,
No. 1, p. 442 (1979).
4. M.O.Bedwell and T.J.Paulus, "A Versatile
Constant-Fraction 100-MHZ Discriminator," IEEE
Trans. Nucl. Sci.,
NS-25,
No. 1, p. 86 (1978).
5. M. 0. Bedwell and T. J. Paulus, "A New
Constant-Fraction Timing System with Improved
Time Derivation Characteristics," IEEE Trans. Null
Sci.,
NS-23,
p. 234 (1976).
6. G. F. Knoll, Radiation Detection and
Measurement, John Wiley and Sons, New York,
NY (1979).