LETTER TO DR. PUTHOFF: STANFORD RESEARCH INSTITUTE
CIA-STARGATE
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The document consists of several pages from the CIA's Stargate program. The content includes information about photodetectors, silicon charged-particle detectors, operating temperature, and the nature and propagation of light. It discusses the effects of radiation damage on detectors, the relationship between operating temperature and detector performance, and the electromagnetic spectrum. It also includes a graph showing the relative eye sensitivity to different wavelengths of light.
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0.8 1 1.2
WAVELENGTH (MICRONS)
ands `photodetectors.
(After Melchior and Lynch, Ref. 39.)
Fig. 25
(a) Equivalent circuit and
(b) noise equivalent circuit of a photodlode, where R Is the series resistance and C Is the
junction capacitance.
(After DiDomenico and Svelto, Ref. 35.)
CPYRGHT
4 Photodetectors
available power for the photo
Pav = I II PD\(0) I
It is interesting to compare F
For a typical photodiode with
a photoconductor with the sar
available power from the phot
from the photoconductor.
The signal-to-noise perforn
equivalent noise circuit shown
noise source due to the serie,,
source. The signal-to-noise ray
Comparing Eq. (44) with Eq.
at high-level detection where
SNR is comparable; at low-leN
however, the SNR of the pho
B. The p-f-n Photodiode
depletion-layer photodetector.
(the intrinsic layer) can be tai
frequency response. A typical
Fig. 26(a). Absorption of ligh
pairs. Pairs produced in the de
will eventually be separated by
external circuit as carriers drif
Under steady-state conditioi
biased depletion layer is given
where Jd, is the drift current
region and Jd)ff is the diffusior
side the depletion layer in the 1
reverse-biased junction. We
assumptions that the thermal g
surface n layer is much thinn,
electron generation rate is givt
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all
CPYRGHT
SILICON CHARGED-PARTICLE DETECTORS
characteristics. The detector changes include in-
creased noise and changes in voltage drop across the
load resistor, which require adjustments to the applied
bias voltage, which in turn change the electric-field
strength. Thus carrier trapping and increased detector
noise are degrading to energy resolution.
Resolution degradation appears as a broadening of the
response for a monoenergetic source. With increasing
doses of neutrons, charged particles, or fission frag-
ments, the low-energy side of the response peak may
begin to show a definite secondary peak. Continued
irradiation results in further broadening, until, in ex-
treme cases, the multiple peaks may merge com-
pletely. Electron bombardment tends to increase leak-
age current, resulting in excess detector noise, which
broadens response peaks. Some of these damage ef-
fects may undergo a degree of annealing, but there is
always a significant residual deterioration after a suffi-
cient dose has been accumulated.
Partially depleted detectors are more susceptible than
are fully depleted devices to deterioration from radia-
tion damage. Radiation damage for different types of
detectors are compared in Table 2, which gives the
dose for various particles to significantly deteriorate
the detectors.
OPERATING TEMPERATURE
As a rule of thumb, increasing the operating tempera-
ture of a charged-particle detector causes the leakage
current to incur lase b a factor of 3 for each 100C rise.
su_ (tinsi in a noise-widt increase QL3,{ pr ate ,1.J
ke~L [j e. The upper temperature limit is deter-
mined by the maximum acceptable noise or by the
ultimate breakdown of the detector (usually between
45 and 55?C). The effects of high-temperature break-
down are permanent and are not covered by the war-
ranty terms. An additional effect is the shift in detector
bias caused by the higher leakage current. This leak-
age current increases the voltage drop across the se-
ries bias resistor, thus lowering the bias voltage across
the detector. When high-temperature operation is
necessary, a constant sensitive depth is maintained
over the entire operating temperature range only if a
totally depleted detector is used with sufficient
overbias to compensate for the drop across the series
bias resistor, which should be as small as possible
(usually 1 to 3 M&2 is adequate).
Decreasing the operating temperature of the detector
reduces junction noise and leakage current. However,
the capacitance of the device is a constant limiting
parameter of the system noise. Another limitation to
successful operation at low temperatures is the expan-
sion coefficient of the detector's component parts. The
expansion coefficient is similar for silicon and for lavite,
the ring in which the silicon wafer is mounted, but is
quite different for the bonding epoxy. Therefore at very
low temperatures the epoxy may crack, causing exces-
sive noise or loss of contact. The probability of low-
temperature damage increases with detector size. For
cooled operation, detectors fabricated with cryogenic
epoxy may be special ordered from ORTEC.
Another effect of decreasing the operating tempera-
ture of a silicon detector is an increase of the average
energy necessary to create an electron-hole pair, e.
Due to a widening of the bandgap of silicon in the
temperature range from 300 K to 80 K, e increases,
from 3.62 eV to 3.72 eV. A result of this increase is an
apparent shift in energy of a measured spectroscopic
line. For instance, Fig. 8 shows the apparent peak shift
of the 5.477-MeV 241Am alpha particle peak in the 4.2-
K to 320-K temperature range measured with silicon
charged-particle detectors.
SHOCK AND VIBRATION
Many ORTEC surface-barrier detectors have been
subjected to the shock and vibration tests required for
Table 2. Comparison of Radiation Damage in Silicon and Germanium Particle Detectors
Surface barrier
Diffusion junction
Si(Li)
Ge(Li)
1013
1013
1012
A
Pa
lpha
rticles
Fi
Fra
ssion
gments
1012
1010
109
108
10'2
1010
101,
108
1011 10
8-109
108-109
Ohl
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S - Chap. 39
-inp 'urged. (a) Show that the
olial volume. (b) Show that
ated by integrating the Poynting
0 equal to the rate at which the
energy density for all points within
-he Poynting vector point of view,
gh the wires but through the space
we must first find B, which is the
_ing the charging process; see Fig.
CPYRGHT
Nature and Propagation of Light
CHAPTER 40
40-1 Light and the Electromagnetic Spectrum
Light was shown by Maxwell to be a component of the electromagnetic
spectrum of Fig. 40-1. All these waves are electromagnetic in nature and
have the same speed c in free space. They differ in wavelength (and thus
in frequency) only, which means that the sources that give rise to them and
the instruments used to make measurements with them are rather different.*
The electromagnetic spectrum has no definite upper or lower limit. The
labeled regions in Fig. 40-1 represent frequency intervals within which a
common body of experimental technique, such as common sources and com-
mon detectors, exists. All such regions overlap. For example, we can pro-
duce radiation of wavelength 10-3 meter either by microwave techniques
(microwave oscillators) or by infrared techniques (incandescent sources).
I V
Frequency, cycles/sec
102 104 106 106 1010 1016 101 10u 1018 10'0 10 6
Power Mkrwwaws _ bis -- X-nws
Radio _ Ultravio et _ mma rays
1 I I 1 I I I I 1 1 I I I I 1 I I I 1 I I
10-14
106 i04 10a 1 10'2 10-4 10-e 10-e 10-10 10 12
Wavelength, meters
pig. 40-1 The electromagnetic spectrum. Note that the wavelength and frequency
scales are logarithmic.
* For a report of electromagnetic waves with wavelengths as long as 1.9 X 107 miles
the student should consult an article by James Heirtaler in the Scientific American for
March 1962.
A p p roj e1ease
994 NATURE AND PROPAGATION OF LIGHT Chap. 4(
Wavelength, m?
Fie. 40-2 The relative eye sensitivity of an assumed standard observer at different wave-
lengths for normal levels of illumination. The shaded areas represent the (continuously
graded) color sensations for normal vision.
"Light" is defined here as radiation that can affect the eye. Figure 40-2,
which shows the relative eye sensitivity of an assumed standard observer to
radiations of various wavelengths, shows that the center of the visible region
is about 5.55 X 10_7 meter. Light of this wavelength produces the sensa-
tion of yellow-green. *
In optics we often use the micron (abbr. u) the millimicron (abbr. m?),
and the Angstrom (abbr. A) as units of wavelength. They are defined from
1 ? = 10-8 meter
1 m? = 10--9 meter
1 A = 10-10 meter.
Thus the center of the visible region can be expressed as 0.555 z, 555 m?, or
55,50 A. ...........,.~...~,~,~
The Iiiiiits of the visible spectrum are not well defined because the eye
sensitivity curve approaches the axis asymptotically at both long and short
wavelengths. If the limits are taken, arbitrarily, as the wavelengths at
which the eye sensitivity has dropped to 10/, of its mA.x;m?m val?o +I,---
The eye can detect is ion beyond these limits if it is intense enoughIn
many experiments in physics one can use photographic plates or light-sensi-
tive electronic detectors in place of the human eye.
*Bee "Experiments in Color Vision" by Edwin H. Land, Scientific American, May
1959, and especially "Color and Perception: the Work of Edwin Land in the Light of
Current Concepts" by M. H. Wilson and R. W. Brocklebank, Contemporary Physics,
December 1961, for a fascinating discussion of the problems of perception and the distinc-
tion between color as a characteristic of light and color as a perceived property of objects.