Study shows the pulse-height degradation of gamma ray spectra in germanium/lithium detectors to be due to electron interaction in the dead layers that exist in all semiconductor detectors. A pulse shape discrimination technique identifies and eliminates these defective pulses

Larsen, R. N.

Strauss, M. G.

English

NASA Technical Reports Server

December 1969
	 Brief 69-10767 
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Pulse-Height Defect Due to Electron Interaction 
in Dead Layers of Ge(Ei) Gamma-Ray Detectors 
Study of the pulse-height degradation of gamma-
ray spectra in germanium detectors has shown it to be 
due to electron interaction in the "dead layers" that 
exist in all semiconductor detectors (see reference). 
These interactions give rise to pulses having slower 
rise times than ordinary or normal pulses. Effective 
means to eliminate this problem are discussed. 
This effect, of the amplifier time constants being 
shorter than the rise time of the pulse, had been 
observed previously and labeled the "ballistic deficit." 
However, this study appears to be the first systematic 
attack on the problem of the origin of the slow-rising 
pulse and the ballistic deficit. 
Gamma rays giving rise to ionizing electrons, in the 
dead layers of Ge(Li) detectors, result in the slow 
pulses of reduced amplitude that degrade energy 
spectra and coincidence resolution. These pulse-height 
defects are not inherent in the detectors' properties, 
but result from current detector-fabrication techniques. 
The factors affecting spectral degradation include 
the thickness of the dead layer relative to the com-
pensated region, the field intensity in the dead layer, 
and the pulse-shaping time constants in the main 
amplifier. 
It is shown that, when internal-conversion electrons 
from 207 Bi (about I MeV) impinge on a detector with 
a thin dead layer of 10i, the peaks characteristic of 
the K and L electrons are well defined; however, when 
they impinge on a detector with a dead layer of 0.3 
mm, the electron spectrum shows no features except a 
continuum that rises at the low-energy end. The distri-
bution of rise times, resulting from the interaction of 
conversion electrons in the latter detector, exhibits 
a peak indicating abundance of slow pulses.
A similar peak is apparent in the rise-time distri-
bution from gamma rays in the continuum of the 
energy spectrum, but not from gamma rays in the 
full-energy peak. This fact suggests that a significant 
fraction of the pulses in the continuum is due to 
electron and/or positron interaction in the dead 
layers. 
A pulse-shape-discrimination technique was de-
veloped whereby defective pulses are identified and 
rejected. Gamma-ray spectra from a single Ge(Li) 
detector and from a Ge(Li)—Nal(Tl) three-crystal 
pair spectrometer were studied. Comparison of these 
spectra with and without rejection of slow pulses 
shows that, in an 11-mm-thick detector, the portion 
of the continuum due to defective pulses approaches 
50%. 
The discrimination technique is described, and 
results of several experiments are presented. 
Reference: 
1. Strauss, M. G.; Larsen, R. N.: Pulse Height De-
fect Due to Electron Interaction in the Dead Layers of 
Ge(Li) -y-Ray Detectors. Argonne National Labora-
tory, May 1967. 
Notes: 
1. Organizations using high-resolution spectroscopy 
may be interested. 
2. Inquiries concerning this information may be di-
rected to:
Office of Industrial Cooperation 
Argonne National Laboratory 
9700 South Cass Avenue 
Argonne, Illinois 60439 
Reference: B69-10767
(etit ucd verleal) 
This document was prepared under the sponsorship of the Atomic 
Energy Commission and/or the National Aeronautics and Space 
Administration. Neither the United States Government nor any per-
son acting on behalf of the United States Government assumes any
liability resulting from the use of the information contained in this 
document, or warrants that the use of any information, apparatus, 
method, or process disclosed in this document may not infringe pri-
vately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19690000653 2020-03-16T18:34:34+00:00Z
Patent status: 
Inquiries concerning rights to commercial use of 
this information may be made to: 
Mr. George H. Lee, Chief 
Chicago Patent Group 
U.S. Atomic Energy Commission 
Chicago Operations Office 
9800 South Cass Avenue 
Argonne, Illinois 60439 
Source: M. G. Strauss and R. N. Larsen

Electronics Division
(ARG- 10362)
. 
Brief 69-10767 Category 02


Pulse-height defect due to electron interaction in dead layers of Ge/Li/ gamma-ray detectors

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