Using here the CS-137 @ 5 uCi, which gives off some Betas
but
primarily a 661.6keV Gamma.
A homemade aluminum capsule filters the Beta, and yields a pure
Gamma source.
Another isotope frequently used to test GM tubes is Co-60.
For testing, a smoke detector ( assembled/intact) makes another
good Gamma source @ about 60keV. Do not remove or expose Am241
pellet inside as this is a Federal offense according to the NRC.
The metal housing screens out all the Alphas, leaving only the Gamma
ray coming through.
The only legal Alpha disc source is Po-210, but you can also own any
of the natural Thorium sources. I use both Th230
and Th232.
The other disc used here is Sr-90 @ .1 uCi, which is a Beta emitter.
Testing all probe tubes on the same calibrated Eberline RM-20 Geiger
Counter set at 40 mV threshold
( not that it matters for GM tubes-all pulses are strong and the
same usually 5+ Volts) , the following readings were obtained
( Those not mentioning any Alpha are not Alpha capable. Any windows
must be open for Beta detection):
THIN SIDE WALL GM cylindrical or Hot-Dog Tubes plus Pancake for
comparison:
Tube Type 1B85
Window Open/CS-137 90,000 cpm
Window Closed CS-137 9,000 cpm
Window Open SR-90 27,000 cpm
Tube Type 6306
Window Open/CS-137 20,000-35,000 cpm Asymmetric
Window Closed CS-137 17,000 cpm
Window Open SR-90 7,000 cpm
Tube Type LND725
Window Open/CS-137 27,000 cpm
Window Closed CS-137 6,000 cpm
Window Open SR-90 4,000 cpm
Tube Type Raytheon #1021
Window Open/CS-137 90,000 cpm
Window Closed CS-137 9,000 cpm
Window Open SR-90 25,000 cpm
Tube Type 6993 Black
Window Open/CS-137 44,000 cpm
Window Closed CS-137 4,000 cpm
Window Open SR-90 13,000 cpm
Tube Type 6993 yellow
Window Open/CS-137 41,000 cpm
Window Closed CS-137 4,400 cpm
Window Open SR-90 11,000 cpm
Tube Type T/A P-10 (LND 721)
Window Open/CS-137 20,000 cpm
Window Closed CS-137 4,500 cpm
Window Open SR-90 8,000 cpm
Tube Type Pancake LND 7311
Window Open/CS-137 170,000 cpm
Window Closed CS-137 14,000 cpm
Window Open SR-90 45,000 cpm
All the above tests ran @ 900V DC, the "standard" operating voltage
of all
these series of GM tubes.
1B85 and 6306 are the thin aluminum housing hot-dog tubes typically
used in 1950's era Geiger Counters, like Precision and Heathkit.
They show
remarkable sensitivity, but are very fragile.
The LND 725 is the modern equivalent, but has a sturdy metal(
stainless steel)
housing and is used in GEO-270hp, Ludlum 44-6 and Eberline HP-270
hand probes.
All three of these tubes have
a single pin at the bottom for anode connection, and the body itself
is the
cathode connection. In the
photo (FILES section) you can see the disassembled EberlineHP-270
hand probe housings with sliding
Beta window.
Three pin connectors are used on the 1021, 6993, and the
T/A 721 tube. All these are metal hot-dogs except the Raytheon 1021
which is glass.
Typical CDV-700 comes with a 6993 tube in the hand probe, the LND
720 being the modern commercial equivalent.
END-WINDOW type tubes:
Here we are having fun (Photo in FILES section) testing end-window
GM tubes on the same setup we used
before to test the sidewall and pancake tubes.
Test source #1 is a CS-137 @ 5 uCiTest Source # 2 is a Sr-90 @ .1
uCi, which is a Beta emitter.
Testing some probe tubes on the same calibrated Eberline RM-20
Geiger Counter set at 40 mV threshold,
the following readings were obtained:
Probe type Ludlum 44-7*
Window Open/CS-137 90,000 cpm
Window Closed CS-137 5,000 cpm
Window Open SR-90 20,000 cpm
Probe Type LND 7232 *
Window Open/CS-137 90,000 cpm
Window Closed CS-137 5,000 cpm
Window Open SR-90 23,500 cpm
Probe Type Technical Associates P6A*
Window Open/CS-137 90,000 cpm
Window Closed CS-137 7,000 cpm
Window Open SR-90 25,000 cpm
Probe Type Vic 489-35
Window Open/CS-137 115,000 cpm
Window Closed CS-137 5,500 cpm
Window Open SR-90 24,000 cpm
Note: This tube has a*VERY* thin window and is extraordinarily
sensitive to Alpha particles.
Probe Type PDR27T**
Window Open/CS-137 70,000 cpm
Window Closed CS-137 4,000 cpm
Window Open SR-90 12,000 cpm
Note: this tube is more rugged than most end window types, but the
thicker window yields it insensitive
to Alpha particles.
All the above tests ran @ 900V DC, the "standard" operating voltage
of all these series of GM tubes,
except for the PDR 27 probe, which we reset the HV to 700V.
* Even though the same tube, LND 723 used in these probes,
differences in
housing and screens makes reading different by a small amount. .
**PDR 27 uses QPL 5979 tube. 700V rating.
Easy to see why I like the Pancake probes so much. Their super
sensitivity
to Alpha, Beta and Gamma radiation is well balanced with a minimum of
background pickup. A homemade Beta window was fashioned from 1/10"
aluminum plate, but you can simply turn the probe over backwards and
use the housing itself as a Beta shield. A pressure difference of
200-220 mm of mercury exist across the mica window, the interior
being of lower pressure than atmospheric. If barometric pressure
changes, as in driving over mountains, or any air transportation,
the window will flex just as the bellows of a barometer do.
Eventually cracks will develop in the brittle mica, causing ingress
of air, which will destroy the tube. Any kind of puncture will
immediately render the tube window broken as well, so extreme
caution is needed in transportation and handling. I find them
reasonably rugged, and have lost only a few to pressure conditions.
These results again point out the futility of measuring radiation
vs. detecting radiation.
Each of the above probes is giving it's own reading
for a particular type of radiation, and these readings will be
consistent from
tube to tube, of a certain type number. Different type numbers yield
vastly
different readings, making quantitative measurements impossible. The
only way I see to assay a sample, is to have another sample of
Calibrated activity, and then you could only be sure of comparing
the exact same materials.
Specifications for all the above tubes can be found at the LND site.
They
make the tubes used by many different probe manufacturers.
http://www.lndinc.com/product.htm
Pay attention to the Gamma Sensitivity numbers in the specification
sheets and charts
( also reproduced in the FILES section for selected tubes). This
figure gives a clue as to how
sensitive a certain tube is compared to another tube. The number
indicated how many pulses you would
get from a uniform flux of Cs-137 or C0-60, in counts per second/per
mR/H. Multiply time 60 for CPM.
The other factor is energy response, and different tubes will
respond to varying energy levels according mainly to he construction
materials used, and volume of fill gas ( size of tube). In general,
low energy Gammas must be of sufficient strength ( meaning energy
level, not number of disintegrations) to penetrate the housing
material. A Z number is used
to indicate density of any material, based on atomic makeup.Once
inside the tube, lower energy Gammas are much more likely to cause
an interaction, and therefore be counted. At some point as the
energy level increases, the ray will simply pass out of the tube and
not be counted. These facts account for the whipsaw shape of the
energy response curves of all GM tubes. External filtering may be
applied to compensate for this non linear effect, resulting in a
probe that is called" energy compensated". Be aware that this
procedure knocks all the response down to the lowest level, and that
although now nearly perfectly linear, such a probe will give lower
reading than you may be used to from the more common "energy
dependant" probes.
Making a rough estimate of activity may be found by applying this
formula:
@1 meter 1Ci= .381 R
where 1uCi=10^-6 Ci
and using the inverse squared law:
@ 1/2 meter = X4
@1/4 meter = X 16
@ 1/8 meter= X64
@ 1/16 meter = X256
etc.
1 uCi is always equal to 3.7 X 10^4 DPS ( disintegrations per
second) or 2.22 X10^6 DPM no matter what type of
radiation is involved.
http://www.sizes.com/units/curie.htm
http://www.radcon.arizona.edu/training/RSPC-CH.pdf
When the term 4Pi is used, it refers to disintigrations in all
directions, as in a shpere. Most probes can only see from one
direction and as such are 2Pi ( 1/2 of a shpere). GEOMETRY is the
term used to indicate the area that the radiation fills in relation
to the probe. Technically it is the angle subtended by the probe.
Best geometry is achieved if the probe is 10X it's own diameter away
from the source.
4Pi or near 4Pi can be achieved with hollow probes (as in WELL
probes) where the radioactive sample is placed inside. Liquid
scintillators are also 4Pi, as the sample is inside the liquid.
Have Fun
George Dowell
primarily a 661.6keV Gamma.
A homemade aluminum capsule filters the Beta, and yields a pure
Gamma source.
Another isotope frequently used to test GM tubes is Co-60.
For testing, a smoke detector ( assembled/intact) makes another
good Gamma source @ about 60keV. Do not remove or expose Am241
pellet inside as this is a Federal offense according to the NRC.
The metal housing screens out all the Alphas, leaving only the Gamma
ray coming through.
The only legal Alpha disc source is Po-210, but you can also own any
of the natural Thorium sources. I use both Th230
and Th232.
The other disc used here is Sr-90 @ .1 uCi, which is a Beta emitter.
Testing all probe tubes on the same calibrated Eberline RM-20 Geiger
Counter set at 40 mV threshold
( not that it matters for GM tubes-all pulses are strong and the
same usually 5+ Volts) , the following readings were obtained
( Those not mentioning any Alpha are not Alpha capable. Any windows
must be open for Beta detection):
THIN SIDE WALL GM cylindrical or Hot-Dog Tubes plus Pancake for
comparison:
Tube Type 1B85
Window Open/CS-137 90,000 cpm
Window Closed CS-137 9,000 cpm
Window Open SR-90 27,000 cpm
Tube Type 6306
Window Open/CS-137 20,000-35,000 cpm Asymmetric
Window Closed CS-137 17,000 cpm
Window Open SR-90 7,000 cpm
Tube Type LND725
Window Open/CS-137 27,000 cpm
Window Closed CS-137 6,000 cpm
Window Open SR-90 4,000 cpm
Tube Type Raytheon #1021
Window Open/CS-137 90,000 cpm
Window Closed CS-137 9,000 cpm
Window Open SR-90 25,000 cpm
Tube Type 6993 Black
Window Open/CS-137 44,000 cpm
Window Closed CS-137 4,000 cpm
Window Open SR-90 13,000 cpm
Tube Type 6993 yellow
Window Open/CS-137 41,000 cpm
Window Closed CS-137 4,400 cpm
Window Open SR-90 11,000 cpm
Tube Type T/A P-10 (LND 721)
Window Open/CS-137 20,000 cpm
Window Closed CS-137 4,500 cpm
Window Open SR-90 8,000 cpm
Tube Type Pancake LND 7311
Window Open/CS-137 170,000 cpm
Window Closed CS-137 14,000 cpm
Window Open SR-90 45,000 cpm
All the above tests ran @ 900V DC, the "standard" operating voltage
of all
these series of GM tubes.
1B85 and 6306 are the thin aluminum housing hot-dog tubes typically
used in 1950's era Geiger Counters, like Precision and Heathkit.
They show
remarkable sensitivity, but are very fragile.
The LND 725 is the modern equivalent, but has a sturdy metal(
stainless steel)
housing and is used in GEO-270hp, Ludlum 44-6 and Eberline HP-270
hand probes.
All three of these tubes have
a single pin at the bottom for anode connection, and the body itself
is the
cathode connection. In the
photo (FILES section) you can see the disassembled EberlineHP-270
hand probe housings with sliding
Beta window.
Three pin connectors are used on the 1021, 6993, and the
T/A 721 tube. All these are metal hot-dogs except the Raytheon 1021
which is glass.
Typical CDV-700 comes with a 6993 tube in the hand probe, the LND
720 being the modern commercial equivalent.
END-WINDOW type tubes:
Here we are having fun (Photo in FILES section) testing end-window
GM tubes on the same setup we used
before to test the sidewall and pancake tubes.
Test source #1 is a CS-137 @ 5 uCiTest Source # 2 is a Sr-90 @ .1
uCi, which is a Beta emitter.
Testing some probe tubes on the same calibrated Eberline RM-20
Geiger Counter set at 40 mV threshold,
the following readings were obtained:
Probe type Ludlum 44-7*
Window Open/CS-137 90,000 cpm
Window Closed CS-137 5,000 cpm
Window Open SR-90 20,000 cpm
Probe Type LND 7232 *
Window Open/CS-137 90,000 cpm
Window Closed CS-137 5,000 cpm
Window Open SR-90 23,500 cpm
Probe Type Technical Associates P6A*
Window Open/CS-137 90,000 cpm
Window Closed CS-137 7,000 cpm
Window Open SR-90 25,000 cpm
Probe Type Vic 489-35
Window Open/CS-137 115,000 cpm
Window Closed CS-137 5,500 cpm
Window Open SR-90 24,000 cpm
Note: This tube has a*VERY* thin window and is extraordinarily
sensitive to Alpha particles.
Probe Type PDR27T**
Window Open/CS-137 70,000 cpm
Window Closed CS-137 4,000 cpm
Window Open SR-90 12,000 cpm
Note: this tube is more rugged than most end window types, but the
thicker window yields it insensitive
to Alpha particles.
All the above tests ran @ 900V DC, the "standard" operating voltage
of all these series of GM tubes,
except for the PDR 27 probe, which we reset the HV to 700V.
* Even though the same tube, LND 723 used in these probes,
differences in
housing and screens makes reading different by a small amount. .
**PDR 27 uses QPL 5979 tube. 700V rating.
Easy to see why I like the Pancake probes so much. Their super
sensitivity
to Alpha, Beta and Gamma radiation is well balanced with a minimum of
background pickup. A homemade Beta window was fashioned from 1/10"
aluminum plate, but you can simply turn the probe over backwards and
use the housing itself as a Beta shield. A pressure difference of
200-220 mm of mercury exist across the mica window, the interior
being of lower pressure than atmospheric. If barometric pressure
changes, as in driving over mountains, or any air transportation,
the window will flex just as the bellows of a barometer do.
Eventually cracks will develop in the brittle mica, causing ingress
of air, which will destroy the tube. Any kind of puncture will
immediately render the tube window broken as well, so extreme
caution is needed in transportation and handling. I find them
reasonably rugged, and have lost only a few to pressure conditions.
These results again point out the futility of measuring radiation
vs. detecting radiation.
Each of the above probes is giving it's own reading
for a particular type of radiation, and these readings will be
consistent from
tube to tube, of a certain type number. Different type numbers yield
vastly
different readings, making quantitative measurements impossible. The
only way I see to assay a sample, is to have another sample of
Calibrated activity, and then you could only be sure of comparing
the exact same materials.
Specifications for all the above tubes can be found at the LND site.
They
make the tubes used by many different probe manufacturers.
http://www.lndinc.com/product.htm
Pay attention to the Gamma Sensitivity numbers in the specification
sheets and charts
( also reproduced in the FILES section for selected tubes). This
figure gives a clue as to how
sensitive a certain tube is compared to another tube. The number
indicated how many pulses you would
get from a uniform flux of Cs-137 or C0-60, in counts per second/per
mR/H. Multiply time 60 for CPM.
The other factor is energy response, and different tubes will
respond to varying energy levels according mainly to he construction
materials used, and volume of fill gas ( size of tube). In general,
low energy Gammas must be of sufficient strength ( meaning energy
level, not number of disintegrations) to penetrate the housing
material. A Z number is used
to indicate density of any material, based on atomic makeup.Once
inside the tube, lower energy Gammas are much more likely to cause
an interaction, and therefore be counted. At some point as the
energy level increases, the ray will simply pass out of the tube and
not be counted. These facts account for the whipsaw shape of the
energy response curves of all GM tubes. External filtering may be
applied to compensate for this non linear effect, resulting in a
probe that is called" energy compensated". Be aware that this
procedure knocks all the response down to the lowest level, and that
although now nearly perfectly linear, such a probe will give lower
reading than you may be used to from the more common "energy
dependant" probes.
Making a rough estimate of activity may be found by applying this
formula:
@1 meter 1Ci= .381 R
where 1uCi=10^-6 Ci
and using the inverse squared law:
@ 1/2 meter = X4
@1/4 meter = X 16
@ 1/8 meter= X64
@ 1/16 meter = X256
etc.
1 uCi is always equal to 3.7 X 10^4 DPS ( disintegrations per
second) or 2.22 X10^6 DPM no matter what type of
radiation is involved.
http://www.sizes.com/units/curie.htm
http://www.radcon.arizona.edu/training/RSPC-CH.pdf
When the term 4Pi is used, it refers to disintigrations in all
directions, as in a shpere. Most probes can only see from one
direction and as such are 2Pi ( 1/2 of a shpere). GEOMETRY is the
term used to indicate the area that the radiation fills in relation
to the probe. Technically it is the angle subtended by the probe.
Best geometry is achieved if the probe is 10X it's own diameter away
from the source.
4Pi or near 4Pi can be achieved with hollow probes (as in WELL
probes) where the radioactive sample is placed inside. Liquid
scintillators are also 4Pi, as the sample is inside the liquid.
Have Fun
George Dowell