Circuit for protecting photomultiplier equipment from current surges which occur when exposed to brilliant illumination is discussed. Components of circuit and details of operation are provided. Circuit diagram to show action of blanking pulse on zener diode is included

Mcclenahan, J. O.

English

NASA Technical Reports Server

September 1972	 B72-10561 
NASA TECH BRIEF 
Ames Research Center 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Photomultiplier Blanking Circuit 
It is necessary to protect a photomultiplier from 
the destructive current surges which occur when it is 
exposed to brilliant illumination. For example, a 
photomultiplier is operated at very high voltages in 
order to sense the very low light levels in the pre-
cursor region in shock-tube measurements but, since 
the radiative intensity is many decades higher when 
the incident shock wave arrives in the field of view, 
the photomultiplier must be shielded from the dam-
aging effects of the radiant pulse. 
In the past, intense radiant pulses have been 
blocked by mechanical shutters, or the high voltage 
input to the photomultiplier has been momentarily 
short circuited. Unfortunately, very fast shutter clo-
sures (i.e., less than 1 millisecond) are not readily 
achieved and, as a result, complicated shuttering 
devices have had to be developed. For example, to 
obtain closures within 10 microseconds or less, powder 
charges or high-current magnetic pulses are needed 
to force a shutter to move into the field of view of the 
detector system. Electronic systems which short cir-
cuit high voltage power supplies in microseconds also 
require complex circuitry and devices, such as igni-
trons; moreover, the power supply must be designed 
to withstand a short circuit at its output. 
The photomultiplier blanking circuit shown in the 
diagram makes use of the fact that the overall gain 
of the photomultiplier is the product of the gain of 
each stage of multiplication in the dynode chain. 
Thus, if the gain of one stage is reduced to zero, the 
overall gain is also brought to zero. The circuit 
operates as follows: With the high voltage supply on 
and the silicon controlled-rectifier (SCR) not conduct-
ing, the photomultiplier is biased normally and has
the high gain required to measure low levels of radi-
ant energy. When a blanking pulse is applied, the 
SCR is triggered into conduction and the voltage 
between the cathode and the second dynode drops 
put 
Blanking P:
	
. Input  
from its previous value (ordinarily between 100 to 
400 volts) to less than about S volts. Now, photo-
electrons emitted by the photocathode are not acceler-
ated toward the first dynode, and the few that manage 
to arrive have insufficient energy to cause reemission 
from this dynode.
(continued overleaf) 
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The circuit shown in the diagram provides for the 
short-circuiting of two stages to provide double assur-
ance that no electrons propagate to the subsequent 
stages which are still at full accelerating potential. 
Measurement of the voltage between the photocath-
ode and the second dynode indicates that sensitivity 
to light is eliminated within 5 microseconds after 
application of the blanking pulse; however, the sud- 
den increase in voltage across the last dynode appears 
as a 2-millisecond spike at the output (regardless of 
illumination). The increase in power supply current 
resulting from the shorting of two stages in the 
dynode resistor chain is only about 20 percent; thus, 
a special power supply is not required. The pulse 
required to trigger the blanking circuit is about 20 
volts, and it can be conveniently obtained from the 
positive gate-pulse of most oscilloscopes.
Note: 
No additional documentation is available. Specific 
questions, however, may be directed to: 
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: B 72-10561 
Patent status: 
Inquiries concerning rights for the commercial use 
of this invention should be addressed to: 
NASA Patent Counsel 
Mail Code 200-1A 
Ames Research Center 
Moffett Field, California 94035 
Source: James 0. McClenahan 
Ames Research Center
(ARC-10593) 
B72-10561	 Category 01


Photomultiplier blanking circuit

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