Effect of crystal growth direction on the energy resolution of high-purity germanium detectors Page: 4 of 10
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ferentially etched detector surfaces are
shown. Detectors 463-6.3 and 281-8.8 are
both from  crystals, average EPD
5 x 103 cm-2 and 1.1 x 104 cm-2 respec-
tively. Device 499-9.4, from a  crys-
tal, has an average EPD of 1.0 x 104 cm-2.
Device 281-8.8 was cut square to eliminate
high EPD regions near the edge, making
the average EPD more nearly that of 499-9.4.
EPD at lineages (appearing as bright lines
in photographs) are 4.5 x 104 cm-2(463-6.3),
1 x 105 cm-2 (499-9.4 and 281-8.8).
An average dislocation density was computed by
counting the number of etch pits in several repre-
sentative areas and then figuring a weighted mean.
This procedure was required since dislocations in
 material develop in a much more non-uniform
pattern than in  grown germanium and because
lineages represent a high etch pit density for only
a very small area. This latter constraint becomes
important when one considers that the critical etch
pit density of 104 cm-2 could be reached by having
only isolated dislocations, only lineages, or some
combination of the two.
Following this averaging procedure, it was found
that the mean number of etch pits in a  crystal
increased considerably more slowly from seed end to
"tail" end than in a  direction crystal.
463-6.3 281-8.8 499-9.4
2.OkeV- 2kFWHM 1.8FkeVWHM
2. - 2keV .+ I .kV .
* . *'
FW0ioM 5.5keV. FWOM
3.7keVb * 3.6keV+1 _
a) .. b) C)
Fig. 2. Collimated 59.6'keV gamma-rays from 214Am
were scanned across lineages in both detec-
tors. The counts in each peak were accumu-
lated for four minutes and plotted on a
logrithmic scale. The amplifier setting
was more sensitive for detector 499-9.4 to
exaggerate any tailing, giving rise to the
apparent difference in peak width as com-
pared to 281-8.8. Peaks a) and c) were
collected at the lineages (EPD -105 cm-2),
peaks b) and d) were collected at a refer-
ence position (EPD ~104 cm-2).
We have found that at a position near the "tail"
representing 90% of the solid fraction of the crystal
the average EPD for a  crystal may only be
slightly > 104 cm-2, whereas at a similar position a
 grown crystal may have an EPD > 4 x 104 cm-2.
The only exceptions to this observation have been a
few  crystals where a sudden, catastrophic change
to severly disturbed crystallography has been noted.
Gamma-Ray Spectrometer Performance
The significance of the manner in which disloca-
tions are distributed is shown in Fig. 1. Photograph
la shows the shape of the 1.17 MeV 6OCo line for a
detector (463-6.3) made from  Ge whose average
EPD is ~"5 x 103 cm-2, but which contains lineages
where the EPD is - 4.5 x 104 cm-2. Nevertheless, the
device exhibits a FWHM of 2.0 keV with no visible
asymmetry in the peak. Scanning the lineage with
24Am 59.6 keV gamma radiation shows an increase in
the FWHM and peak asymmetry in those high-density
areas. This type of charge trapping is seen more
clearly in Fig. lb for detector 281-8.8. This detec-
tor has an EPD of - 1.1 x 104 cm-2 and its 60Co
resolution suffers seriously as predicted by Glasow
and Haller. By contrast, the third device 499-9.4
(Fig. lc) has an average EPD of 1.0 x 104 cm-2, but a
FWHM of 1.8 keV and good peak symmetry. This differ-
ence is underscored by comparing the results of scan-
ning with 59.6 keV 241Am gamma-rays as seen in Fig. 2.
Using a 0.5 mm collimator, the radiation was scanned
across the defect in the center of 281-8.8 (a sort of
lineage twisted around itself - EPD -105 cm-2 and
across the shar lineage observed on detector 499-9.4
(EPD -105 cm-2). By collecting counts for four min-
utes at each position and plotting the resultant peak
in a logarithmic scale , the low energy "tailing"
associated with lattice disorder in crystal 281 is
clearly shown. Both the n+ and p+ contacts were scan-
ned and it was found that the tailing was seen only
when irradiating the p+ contact, as found in other
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Hubbard, G.S.; Haller, E.E. & Hansen, W.L. Effect of crystal growth direction on the energy resolution of high-purity germanium detectors, article, October 1, 1978; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc1212986/m1/4/: accessed May 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.