Note: Descriptions are shown in the official language in which they were submitted.
1066433
This invention ~elates to gamma radiation dose-rate meter (Radiac-
meter) and more particularly to a simple, hand held, radiacmeter having
a flashing light display.
This invention is related to the device described and claimed in
A my copending Canadian application~ filed y ~ ~ch /~76
and which describes a radiacmeter with a null deflection readout system.
Radiacmeters are employed to determine gamma radiation dose-rate
information in area6 where a radiation hazard exlsts or may exist. A
standard film badge dosimeter, which measures the cumulative radiation to
which the individual wearer has been exposed, i8 an "after-the-event"
device in that the wearer is not aware of any excessive radiation at the
time of expobure and it is only after the film has been developed that the
hazard can be determined. Hence such a device is of no benefit to an
individual or group of individuals who might be exposed to radiation levels
which impose an immediate risk and whose freedom of action may, therefore,
be limited to moving out of the contaminated area, or taking whatever steps
are possible to find a more sheltered spot. It will be appreciated that,
in peace time, exposure to low dose rates only is normally considered and
such exposure poses no immediate risk to personnel and only a potential
long term risk to those highly specialized personnel who are repeatedly
exposed. In the event of nuclear war however, high dose rates that pose
an immediate risk may well be encountered by a large number of individuals
who are isolated from any control authority and it becomes essential to be
able to determine rapidly that a hazard exists and decide whether evacua-
tion or other measures are possible. A radiscmeter is therefore an essent-
ial piece of equipment. This i8 essentially a "go - no go" sltuation and
high accuracy of the equipment is not required. Small s$ze and ruggedness
are, however, prime considerations, particularly for use in military
survival packs or radiation kits issued to civil authorities or emergency
measures organizations. For extensive civilian use low cost, reliability
and ruggedne6s are of considerable importance and therefore plastic casings
and encapsulation are considered highly desirable.
Many radiacmeters have been developed over the years but such
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instruments tend to be expensiye, heayy, bulky and xelatiYely fragile
because they contain delicate moving parts 80 that they are not entirely
suitable for use in the environmental sit^uations envisioned herein-
above.
It is an ob~ect, therefore, of the present invention to provide
a simple, lightweight, hand held radiacmeter which ig sufficiently rugged
for use in military and civilian ~urvival situations, and which eliminates
all moving parts except for an ON-OFF switch.
Thus, by one aspect of the present invention there is provided
a self contained gamma radiation dose rate meter adapted to be hand held
and comprising:
a substantially rigid, pocket sized casing which contains a power
supply means;
a Geiger-Mueller tube, operatively connected to said power supply;
means for counting pulses transmitted by said Geiger-Mueller tube; and
light means operatively connected to said counting means, providing a
visual display proportional to said counted pulses which is viewable
externally of said casing and which is indicative of a received gamma
radiation dose rate.
The invention will be described in more detail hereinafter with
reference to the drawings in which:
Flgure 1 is an isometric view, partially cut away, of a portable
radiacmeter according to one aspect of this invention;
Figure 2 is a circuit diagram of the counting circuit employed
in the radiacmeter of Figure l;
Figure 3 is a block diagram illustrating the timing of the
flashing light readout of the radiacmeter of Figure l; and
Figure 4, which is located on the sheet containing Figure 1,
is a circuit diagram of the power supply circuit employed in the
radiacmeter of Figure 1,
Turning firstly to Figure 1, there is shown a portable
lQ~6433
radiacmeter 1 which ha6 a lightweight cage, i5 approximately 4 1/8" x
2 1/2~' x 7t8" thick and which weighg between about 6 and 7 1/2 ounces
including the battery. The cage 2 may be fabricated in either metal or
plastic. A metal case, such as an aluminum alloy die-ca~ting, has the
advantage of rigidity and ruggedness but is relatively more expensive to
produce than the preferred thermopla8ticor thermosetting plastic moulded
case. The case 2 is cloeed with an end cap 3 which incorporates an
ON-OFF switch 4 and a battery compartment acces6 cap 5. A screw 6 may
be provided to secure the cap 5 to case 2, or the cap may be designed to
snap over case 2 and resiliently engage therewith in known manner.
The device i8 powered by a single penlight AA ~ize 1.5 V
battery 7 which is retained in a separate compartment 8 at one end of
the case 2. At the opposite end of the case there is provided Geiger-
Nueller (G-M) tube 9 which, if the preferred 18529 series tube is employed,
is designed to Deasure do~e rates over the range 0.1 to 100 R/hr. A
flashing light readout 10, consisting of three different coloured light
emitting diodes (LED) 11, 12,and 13, (Hewlett Packard 4684 series) is
also provided in the case 2 80 that the LEDs are visible eYternally
thereof. The LEDs flash at a rate proportional to the dose rate and
provide a simple, easy-to-read indication of the dose rate level. An
accurate readlng can be obtained by countlng the number of flashes
during a predetenmined countlng period. Each counting perlod, which 18
preferably of about three second duration, is marked by a blank perlod of
about one second during which time all three LEDs are off. Green LED 11
flashes once during each counting period per O.lR/hr of dose rate, orange
LED 12 flashes once per lR/hr of dose rate and red LED 13 flashes once per
lOR/hr as explalned in re tetail hereinunder the reference to Figure 3.
Circuit boards containing the GiM pulse amplification, counting,
timing and power supply circuitry are unted in the caee between the
battery compartment 8 and G~N tube 9.
An effective counting and ti~ing circuit as used in the device
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1066433
of Figure 1 is shown in pigure 2. pulses from the G-M tube 9 are
ampli~ied and fed to four decade-counters Cl-C4 connected in series. The
output of the second decade-counter is al60 fed into a NAND gate G5
inverted (G7) a~d into a constant current stage driving green LED 11.
Similarly the third counter stage drives the orange LED 12 and the
fourth counter feed~ the red LED 13 drlven stage directly. The NAND
gates G5, G6 are connected ~o that only one of the three LEDs can be ON
at any time ao as to limit the peak current drain and also make the
display simpler to read.
The operating voltages required, 500 V for the G-M tube and
3.5 V for the LEDs and counters, sre obtained from a dc-dc converter
which is powered by a suitable sized 1.5 V dry cell. Conveniently a
single pen light AA size 1.5 V battery will suffice as the instrument
is not intended for continuous use and a zinc-carbon cell will provide
for about 20 hours of operation. A manganese-al~aline cell will provide
for at least 30 hours of operation. The circuit diagram of the converter,
which i8 mounted on a suitable board and enclosed in case 2, as previous-
ly indicated, is shown in Figure 4. Transistor Ql is a regulated swing-
ing cho~e 08clllator. Any difference between the 3.5 V output voltage
and the sener voltage is fed to the base of Q2which controlq the base
drive of Qland the power supplied to the output circuits. The battery
current drain varies from about 25 milliamps with no radiation to 35
milliamps with radiation at a battery voltage of 1.5 volts and 60 mllli-
amps at 1 volt for an overscale reading i.e. more than ten red fla~hes
per counting period.
In operation, the counting period is approximately three
seconts and is controlled by a simple multivibrator Gl, G2 which drives
a ~even stage rlpple counter C5. The last two stages of the ripple
counter, fed to a NAND gate G3 and inverter G4, provide a reset pulse
which inhibits the decade counters Cl, C2, C3, C4 for a time equal to
one third of the counting period. At 0.1 R/hr a 18529 G-M tube is
designed to emit about 33 pulses per
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1066433
second or 100 pulses pex counting period. The output stage of each
counter goes posltive after the eighth count and sta~s positive for two
counts, thus the green LED 11 is on for the last 0.6 second of each
countlng perlod at a 0.1 R/hr dose rate. Similarly the orange and red
LEDs 12 and 13 are on for 0.6 second at lR/hr and lOR/hr respectively.
The device is calibrated by ad~u~ting the counting period to give two
red flashes in a known field of 20 R/hr. Second figure accuracy can
then be obtained by counting the flashe~ of the preceding (orange) stage.
For example, at 25 R/hr there will be two red flashes and five orange
flashes after the second red flash and before the blank period. This is
demonstrsted more clearly ln Figure 3. At the 25 R/hr level the G-M
tube pulses at a nominal 25,000 rate in the 3 second counting perlod.
There will therefore be approxi~ately 250 green LED pulses, 25 orange
LED pulses and 2 red LED pulses. It will be appreciated that the green
pulses will be blanked during orange or red pulses and that the orange
pulse~ will be blanked during red pulses, 80 that the vi~ible pulses will
be a- lndicated in Flgure 3. For the purposes of clarity only, 4 green
pulses are illustrated between each orange pulse and lt will be appreciat-
ed that, ln fact, there will be 8 such pulses. Following the counting
period, the timing circuit provides that the reset period is approx1~tely
one second and is signalled by a blank period during which all LEDs
are extinguished.
In order to provide an indication that the instrument is
working in the absence of a radiation field, the red LED is turned on
for 1-2 seconds when the instrument is switched on. Thls is effected,
as shown in Figure 2, by the CR circuit at the input of inverter 4~. As
soon as the red test period has expired the instrument is ready for
routine use and no further warm up period i- required.
It will, of cour~e, be appreciated that Dany difications to
the circuitry and components may be made without departing fro~ the
scope of this invention. For exaDple, although the Hewlett Packard 4684
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1066433
series LED8 used iP tbe iPstX~oent shown in Fi~uxe 1 can be read easily
at night ox in nor~al indoor or dull outdoor lighting conditions, they
cannot be seen clearly in direct sunlight and it may be necessary to
incorporate appropriate i'ilters into the visual sygtem or, in the
alternative to employ larger, brighter LEDs guch a~ the Hewlett Packard
4658 series with attendent modificat~ons to the unit size of the
instrument. Encapsulation of the components of the device contained
within the case i8 also contemplated in order to ensure rigidity,
water proofing ant elimination of long term corrosion and insulation
break-down problems.
Replacing the 18529 G-M tube, or equivalent, with an 18503
G-M tube, or equivalent, enables this instrument to measure dose-rates
over the range 0.1 to lOObR/h.
A solid state detector and amplifier could be used instead
of the 18529 G-M tube and amplifier. This could pexmit four decades of
dose-rate, 0.1 to l,OOOR/h, to be measured. Appropriate modificationc
to the power supply and counting circuit would have to be made.
