A simple, reliable and inexpensive control circuit is described for rapidly reducing the bias voltage across one or more of the dynode stages of a photomultiplier, to substantially decrease its sensitivity to incoming light at those times where excess light intensity might damage the tube. The control circuit comprises a switching device, such as a silicon controlled rectifier (SCR), coupled between a pair of the electrodes in the tube, preferably the cathode and first dynode, or the first and second dynodes, the switching device operating in response to a trigger pulse applied to its gate to short circuit the two electrodes. To insure the desired reduction in sensitivity, two switching stages, the devices be employed between two of the electrode stages, the devices being operated simultaneously to short circuit both stages

Mcclenahan, J. O.

English

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ATTN Of GP
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON, D C 20546
JUL
 1 7 1974
TO: KSI/Scientific & Technical Information Division
Attn: Miss Winnie M. Morgan
FROM: GP/Office of Assistant General
Counsel for Patent Matters
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NASA Patent Case No.
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NOTE - If this patent covers an invention made by a corporate
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Bonnie L. Woerner
Enclosure
https://ntrs.nasa.gov/search.jsp?R=19740019569 2020-03-17T14:59:19+00:00Z
United States Patent
McCleriatfan
[in 3,821,546
[45] June 28, 1974
[54] PHOTOMULT1PLIER CIRCUIT INCLUDING
MEANS FOR RAPIDLY REDUCING THE
SENSITIVITY THEREOF
[75] Inventor- James O. McClenahan, San Jose,
Calif.
[73] Assignee The United States of America as
represented by the Administrator of
the National Aeronautics and Space
Administration, Washington, D.C.
[22] Filed: Nov. 28, 1972
[21] Appl. No.: 310,193
[52] U.S. Cl 250/207, 307/252 L, 307/252 0
[51] IntCl HOlj 39/12
[58] Field of Search 250/207; 307/252 L, 252 Q
[56] References Cited
UNITED STATES PATENTS
2,647,436 8/1953 Shapiro .
2,815,453 12/1957 Colson
2,840,720 6/1958 Van Rennes
2,846,591 8/1958 Valeton
3,543,095 11/1971 Ensmmger
3,614,646 10/1971 Hansen
250/207
250/207
250/207
250/207
250/207
250/207,
Primary Examiner—James W Lawrence
Assistant Examiner— Harold A Dixon
Attorney, Agent, or Firm—Darrell G. Brekke, Armand
G. Morin, Sr.; John R. Manning
[57] ABSTRACT
A photomultiplier tube comprising a plurality of elec-
trodes including a light sensitive cathode, an anode,
and a plurality of dynodes positioned in space-apart
manner along the electron beam path between the
cathode and anode and having a power supply and a
voltage divider for supplying bias voltages across each
successive pair of electrodes along the beam path, at
least one switching device being provided across a pair
of said electrodes, preferably across the cathode and
first dynode or across the first and second dynodes,
which, when activated, will rapidly reduce the bias
voltage across the associated pair of electrodes to near
zero, thereby substantially reducing the sensitivity of
the multiplier to protect it from the effects of exces-
sive light intensity input In a preferred embodiment of
the invention, an SCR is coupled across the cathode
and first dynode and is operated by a blanking or trig-
ger pulse applied to its control gate to rapidly reduce
the bias voltage across this first stage Additional
switching rectifiers may be employed across other
stages of the multiplier to insure the desired reduction
in sensitivity
1 Claim, 3 Drawing Figures
SIGNAL
OUTPUT
' V
(HRSR)
PATENTEDJUN28 1974 3.821.546
-H.V.
POWER
SUPPLY
27
-^ L*
lMHM-l^ j-d>
SIGNAL
OUTPUT
HI-
TRIGGER
PULSE
THRESHOLD
SENSE
42 v ,,
t1 1ms 1ms
Fig.2(a) Fig.2(b)
3,821,546
PHOTOMULTIPLIER CIRCUIT INCLUDING
MEANS FOR RAPIDLY REDUCING THE
SENSITIVITY THEREOF
The invention described herein was made by an em-
ployee of the United States Government and may be
manufactured and used by or for the Government for
governmental purposes without the payment of any
royalties thereon or therefor.
BACKGROUND OF THE INVENTION
In many instances of radiation measurements whit
photomultiplier detectors, it is necessary to isolate or
otherwise shield the photomultiplier device from the
radiation source to protect the photomultiplier from
damage For example, during operation of the detector,
short bursts of very high intensity radiation may im-
pinge on the detector and seriously overload the de-
vice. In shock tube measurements, the light levels are
very minute during the precursor region and the mea-
surement requires a detector with very high sensitivity
or gain. However, when the incident shock wave ar-
rives at the detector, the radiative intensity is suddenly
many decades higher than before and it is necessary
that the detector be able to withstand this sudden surge
or that the detector by protected from this high inten-
sity pulse. *"
In the past, photomultiphers have been protected in
the above type of operation either by means of shutter-
ing mechanically the light beam impinging on the de-
tector or by short circuiting the high voltage at the
input terminals of the photomultiplier by a crow bar
circuit. The mechanical shutter technique is satisfac-
tory so long as very fast shuttering times (eg., less than
1 millisecond) are not needed; the shuttering mecha-
nism becomes relatively complicated at such speeds. In
the case of shock tube measurements, shuttering times
of 10 microseconds or less are required and mechanical
shuttering of the light beam requires the use of very so-
phisticated means such as powder charges or high cur-
rent pulses'moving a shutter into the field of view of the
detector.
The technique of short circuiting the high voltage
input to the detector requires moderately high voltage
devices such as a 3000 V ignitron which will rapidly
short circuit the power supply output terminals. Special
circuitry is required to trigger the ignitron and the
power supply must be designed for or protected from
the effects of the short circuit at its output terminals.
As with the mechanical shuttering approach, this latter
technique requires complex and costly components.
BRIEF SUMMARY OF THE PRESENT INVENTION
The present invention provides a simple, reliable and
inexpensive control circuit for rapidly reducing the bias
voltage across one or more of the dynode stages of a
photomultiplier, to thereby substantially decrease its
sensitivity to incoming light at those times where excess
light intensity might damage the tube The control cir-
cuit comprises a switching device, such as a silicon con-
trolled rectifier (SCR), coupled between a pair of the
electrodes in the tube, preferably the cathode and first
dynode, or the first and second dynodes, the switching
device operating in response to a trigger pulse applied
to its gate to short circuit the two electrodes. To insure
the desired reduction in sensitivity, two switching de-
15
20
25
30
vices may be employed between two of the electrode
stages, the devices being operated simultaneously to
short circuit both stages.
5 BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of a photomultiplier
device incorporating the novel switching device placed
across two of the multiplier stages.
FIGS. 2(a) and (b) are oscilloscope traces of the
10 photomultiplier output illustrating the desired decrease
in sensitivity to incoming light.
DESCRIPTION OF THE PREFERRED
EMBODIMENTS
Referring now to FIG. 1, the typical form of photo-
multiplier device comprises a plurality of electrodes ar-
ranged in sequential manner along an electron beam
path therethrough and including a light sensitive, pho-
toelectron emitting cathode 17, an anode 12, and a
chain of dynodes 13-22. A high voltage, for example of
the order of 1200 V, is coupled from the main power
supply 23 to a voltage divider circuit coupled to the
chain of electrodes and including the series connected
zener diode 24 and resistors 25 and capacitors 26. This
voltage divider provides the desired bias voltage of
about 120 V across each pair of dynodes, the zener
diode insuring a slightly higher, constant bias voltage
between the cathode 11 and the first dynode 13.
A first switching device or silicon controlled rectifier
(SCR) 27 is connected across the cathode 11 and first
dynode 13 while a second SCR 28 is connected across
the first dynode 13 and the second dynode 14. A first
secondary winding 31 of a pulse transformer 32 is cou-
pled to the triggering gate of the first SCR 27 and a sec-
ond secondary 33 on the pulse transformer 32 is cou-
pled to the triggering gate of the second SCR 28. The
primary winding 34 of the pulse transformer is coupled
to a suitable source 35 of a blanking or triggering pulse.
The blanking pulse source 35 is controlled from a volt-
age threshold sensitive circuit 36 coupled to the output
of the multiplier. When a particular maximum voltage
level occurs at the multiplier output indicating an ex-
cessive light intensity, the threshold circuit operates to
initiate the blanking pulse.
In operation, with the high voltage power supply 23
energized and with both SCRs 27 and 28 non-
conducting, the photomultiplier tube is biased in nor-
mal fashion with about 120 V across each neighboring
pair of dynodes. Radiation impinging on the light sensi-
tive cathode 11 will cause photoelectrons to be emitted
therefrom and directed onto the first dynode 13, where
multiple electrons are emitted for each incident elec-
tron, these additional electrons being directed onto the
second dynode 14. Electron multiplication thus takes
place from dynode to dynode until the final electron
collection at the anode 12 in well known manner. The
tube is designed to have a high gain and high sensitivity
and is thus useful for the measurement of very weak ra-
diant intensities. Since the gain of the photomultiplier
is the product of the gain of each stage of multiplication
in the dynode chain, the gain of the device may be re-
duced to zero by reducing the gain of only one stage,
i.e., the stage between two neighboring electrodes, to
zero. In the present invention, this is accomplished by
the SCRs 27 and 28.
When a blanking pulse is applied to the gate of the
SCR 27 between the cathode 11 and the first dynode
45
,-
3,821,546
13 via the pulse transformer 32, the SCR 27 conducts
fully and the voltage between the cathode 11 and the
first dynode 13 drops from its normal value of > 120
V to less than 3 V. Now the photoelectrons that are
being emitted by the cathode 11 are not focused onto 5
the first dynode 13 so that much fewer of these elec-
trons arrive at this dynode 13. In addition, those elec-
trons that do arrive have insufficient energy to cause
reemission from this dynode. The second SCR 28 is
provided to reduce the voltage across the first and sec- 10
ond dynodes 13 and 14 to doubly assure that no elec-
trons will be propagated to subsequent stages of the
multiplier.
Although two switching devices 27 and 28 have been
shown, only one is needed to sufficiently reduce the 15
sensitivity of the photomultiplier to protect it from high
radiant intensity. The preferred position for this one
SCR is between the cathode electrode 11 and the first
dynode 13 since there are a minimum number of emit-
ted electrons at this point and the reduction of the bias 20
voltage at this stage is most effective in preventing
these electrons from striking the first dynode. For the
same reason, the next most preferred stage for the SCR
is between the first dynode 13 and the second dynode
14. Although these two positions are most favored, one 25
or more of the other dynode stages may be short cir-
cuited, as desired, to obtain the decrease in sensitivity.
There is shown in FIG. 2(a) two oscilloscope traces
of the output of a photomultiplier device incorporating 30
the present invention. The lower horizontal trace 41 is
a zero trace taken with no light impinging on the photo-
multiplier. The upper trace 42 is produced with a light
input, the blanking pulse is applied to the SCRs at time
tt and the upper trace deflects to the zero trace indicat- 35
ing the tube sensitivity to light has been eliminated.
This trace would indicate, however, a long settling time
of several milliseconds which would be too slow for
most applications. The measurements shown in FIG.
2(b) was made to determine whether the tube sensitiv-
ity actually dropped to zero this slowly or whether this
effect was due to some other phenomenon. The oscillo-
scope trace of FIG. 2(b) was made with no light im-
pinging on the photosensitive cathode 11 and a blank-
ing pulse applied to the SCRs at time t\. The character-
istic time response of trace 42 and the trace of FIG.
2(b) is seen to be the same, therefore, the tube sensitiv-
ity is not slowly changing. The long time constant pulse
is due to the power supply voltage readjusting itself
across the dynodes not snorted out by the SCRs. The
tube sensitivity actually drops to near zero in the rela-
tively fast time of 5 microseconds.
Although SCRs have been employed as the shorting
devices it should be understood that other suitable
forms of switching devices such as high voltage switch-
ing transistors could be utilized.
What is claimed is:
1. A photomultiplier device comprising a plurality of
electrodes arranged in sequential manner along an
electron beam path therethrough and including a light
sensitive cathode, an anode, and a plurality of dynodes
positioned in spaced-apart manner between the cath-
ode and anode, means including a power supply for
providing bias voltage across successive pairs of said
electrodes, a first switching rectifier coupled between
said cathode and the first dynode in the path between
said cathode and said anode, a second switching recti-
fier coupled between the said first dynode and the sec-
ond dynode in the path between said cathode and said
anode, and means for at times activating said first and
second rectifiers to turn off the device by reducing the
voltage between the electrodes connected to said recti-
fiers to substantially zero and thereby reducing the sen-
sitivity of the photomultiplier to substantially zero.
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Photomultiplier circuit including means for rapidly reducing the sensitivity thereof

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