Probe Electronics

TM 11-6665-245-34

trons are released from it than strike it and they

plastic called light pipes which direct light

are attracted to dynode two because it is more

generated in the phosphor to the photomultiplier

positive than dynode one. This action continues

tube. Alpha particles are detected when the base

through nine dynodes providing more and more

of the probe is placed over or close to materials

electrons as it gets closer to the collector (pin 10).

giving off alpha rays. The alpha particle

The collector absorbs the electrons and they flow

penetrates the phosphor through the slots in the

to ground through resistor R11 forming a negative

base and the aluminized mylar. Each alpha parti-

cle activates the phosphor it penetrates causing a

going voltage pulse which represents the alpha

small speck of light to be generated. The light in

particle which has been detected.

turn is directed through the light pipes to the

(2) Transistor Q1, resistors R12 and R13

photomultiplier where the light is changed to an

and capacitors C3 and C4 are used to counteract

electrical signal.

the effect of the coil cord capacitance on the

b. Auxiliary Probe. The auxiliary probe func-

signal. The output of the transistor Q1 is a low im-

tions in the same manner as the main probe ex-

pedance to match the coil cord. Resistor R12 and

cept that it is smaller and has no light pipes.

capacitor C4 form an integrating circuit to shape

c. X-Ray Probe.

the pulse waveform. Capacitor C3 and resistor

R13 counteract the distributed capacitance of the

NOTE

coil cord to ground.

b. X-Ray Probe. (Figure FO-2). The X-Ray Pro-

In addition to emitting 5.1 MeV alpha

be circuitry contains a PM tube (V2), a pulse

particles, the element Plutonium-239

amplifier (Q10, 11, 12) and a simple channel

also emits x-rays in the energy range of

analyzer (U1,2). Voltages of -950 VDC, + 3.38 VDC,

14 to 21 keV. Because these x-rays are

and -9 VDC are supplied from the radiacmeter

more penetrating than the alpha par-

through the coiled cord. The signal from the x-ray

ticles, they travel further in matter and

probe is also carried to the radiacmeter through

a i r and can be detected at further

the coil cord.

distances from the ground.

t h e l o w e r and

. (1) With

upper

The x-ray probe uses a Ca F 2 (Eu) scintillator-

discriminator (U1) thresholds at typical -2.3 VDC

photomultiplier combination to detect the 14-21

and -6 VDC respectively, the amplifier gain is ad-

keV x-rays from Pu-239. The photomultiplier tube

justed such that the low discriminator triggers for

fits into a tube socket with a voltage divider net-

e n e r g i e s 14 keV and above and the upper

work to supply the dynode voltages. A mu-metal

discriminator triggers for energies 22 keV and

shield surrounds the phototube to protect it from

above. U2 (pins 1, 2, 3, 4, 6) is a one shot which

gain changes due to stray magnetic fields or from

shapes the output pulses. U2 (pins 10, 11, 12, 14) is

changes in orientation with respect to the earth's

a flip-flop which is set by the upper discriminator

magnetic field. The x-ray probe is an assembly

output and thus inhibits the one shot output.

which includes the amplifier-discriminator cir-

(2) Resistors R64, R65, and R66 are

cuits integral to the phototube scintillator hous-

shorted as needed to provide the required voltage

ing. The discriminator is a single channel analyzer

for the photomultiplier tube (V2).

adjusted to detect Pu-239 x-rays.

2-5. Discriminator, Buffer-lnverter Circuit

2-4. Probe Electronics

(Figure 2-5)

(Figures 2-4 and FO-2)

The discriminator and buffer-inverter circuit deter-

a. Main and Auxiliary Probes. The probe cir-

mines the level of pulses which are measured and

cuitry contains a photomultiplier tube (Vi) and

provides a positive going pulse for use by the

transistor Q1. Voltages of minus 950 and minus 9

count rate circuit (U1).

a. In the static condition, Q2 is conducting

are supplied from the radiacmeter through the

and Q3 and Q4 are held cut off. The positive

coil cord. The signal from the main probe (and)

voltage from voltage divider consisting of R17 and

auxiliary probe is also carried to the radiacmeter

R18 biases Q2 on and sets the emitter voltage of

through the coil cord.

Q2 positive. This in turn biases Q3 off which

(1) V1, the photomultiplier tube, receives

biases Q4 off.

light pulses on its cathode. A light pulse releases

b. Negative going pulses are received from

electrons from the cathode which are attracted to

the probe along with noise across resistor R14.

dynode one (pin 1) because it is more positive

Before the negative going pulse can cut off Q2, it

than the cathode. When the electrons from the

must overcome the positive bias set by R18. R18

photo cathode strike the first dynode, more elec-

2-2.1

Change 2