The task and apparatus available
A friend works at a place where they use beta particles to test
material's coating thickness.
The substrate is metal and it has a well known
"reflective" property to beta particles. We call this kind of reflecting
"Backscatter".
At a known distance the metal will reflect a certain
percentage of the beta rays 180 degrees back to a detector. Once this reading is
established as a baseline,
the same metal, this time
with its paint coating applied, is measured. The beta backscatter will be
reduced by the paint, and the thickness of the paint is what
is being tested.
Of course the beta energy must be known and maintained since both penetration
and backscatter change with beta energy.
Once calibrated, very accurate
measurement of quite thin coatings can be tested.
Some industrial systems use this same technique on a paint or coating line to keep tabs on the coating thickness
in real time, adjusting the machinery to keep
it within
tolerance.
This machine is not that sophisticated, being basically a jig
into which samples are periodically checked as part of their QC
program.
The radioactive source is Pm-147, a beta emitter of 0.2447
MeV (244.7 keV) Peak/ 0.0619
(61.9 keV) keV average at
99.99% probability and a T/2 (Half-Life) of
2.623 years. For the rest of this
discussion series we will use the convention of keV
for energy and refer only to the average beta energy.
With such a
relatively short half life, the rad source must be
replaced on a timed basis, every year or so.
One such old source has become
temporarily available to test.
The known factors:
Original activity 75 = microCuries
Present activity = much less than 1/10th
microCuries
The unknown factors:
Date of
manufacture = unmarked or hidden for this test.
The goals of the
testing:
1) Determine if the material is
actually Pm-147 or some other isotope.
2) Determine the present activity
level (in microCuries or nanoCuries)
3) Determine the age
of the source.
Tools available for the experiments (not all will be
used):
1) Pancake probe with analog + digital meter (scaler).
2) Calibrated absorber set (Spectrum
Techniques)
3) Horseshoe magnet
4) Beta
sources:
A) NIST calibrated Tc-99 @ 0.00353 uCi.
(The Source)
B) Uncalibrated Tc-99 disc
C) C-14
discs (Spectrum Techniques)
D) Cl-36 disc (Spectrum Techniques)
E)
Sr-90/Y-90 disc (Spectrum Techniques)
F) NIST calibrated Cs-137/Ba-137 disc
(Spectrum Techniques)
G) Co-60 disc (Spectrum Techniques)
H) I-129 test
tube
I) Kr-85 disc
J) Ni-63 disc
K) Tl-204 discs (Spectrum
Techniques)
We'll go through the various steps in order, explaining
everything along the way. Mistakes will be made but in the end we will have a
sort of lab manual.
The pancake probe as a beta detector:
Pancake probe efficiency as beta detector is a variable depending on
several parameters.
First and most important is the beta energy itself. Beta particles from
nuclear decay are not monoenergetic like gammas and
alphas tend to be, rather a beta stream will represent a RANGE of energies, from
zero up to a particular maximum value for that
isotope.
We often refer to beta energy by either its maximum energy, Emax or its average energy Eav. In
most cases average comes in at about 1/3 max, but in all cases
both figures are well known and published.
Besides the beta energy, probe variations play a part in the absolute
efficiency number for a particular isotope.
Window thickness, variations in the DAG coating, internal gas pressure
all play a part:
Next we must consider the action of deadtime
on any attempt at a true measurement. Gas filled detector tubes need a certain
amount of time to regain their internal equilibrium (ionization status) between
detection events. This so called dead time will affect final readings more and
more as the count rate increases.
At some point, an increase in radiation actually fails to increase the
count rate at all, and if even more radiation is encountered, the GM tube will
fail to report at all, having reached saturation:
http://www.qsl.net/k/k0ff//Deadtime/
Lab Manual
Step 1: Evaluate available beta samples and chose the correct one for
comparison:
Comparative measuring has always proved to give the easiest and best
results in the Home Rad Lab. If two samples are the
same size, consistency, have the same geometry to the probe, and are of the same
isotopes, they can be directly compared. If one of the samples is calibrated,
the unknown sample can be quantified within a reasonable
accuracy.
Since we are testing Pm-147 in this experiment, it's qualities must be charted.
We find from the literature that Pm-147 is a beta emitter with 99.99%
probability with 224.7 keV max beta energy and 61.9
keV average beta energy.
Pm-147 has nil gamma rays.
75 uCi Pm-147 source of unknown
age:
A chart was made of all the available lab
isotopes, only a few of which are NIST traceable
calibrated:
http://www.qsl.net/k/k0ff//Pancake%20Probe%20Beta%20Efficiency/Beta%20Sources%20Available.txt
Ideally a calibrated Pm-147 test disc would make the measurement easy,
but we had no such disc. It was decided to compare to Tc-99, also a pure beta
emitter, and we do have one calibrated disc.
Beta Emission Products: Tc-99
Fraction .999990 Maximum Energy (MeV) 0.204200
Average Energy (MeV)
0.084600
A second disc is also available, labeled but not
calibrated:
With an extremely long half life of 213 thousand years, no effort was
made to calculate the present day activity of the calibrated Tc-99 disc.
It has been stored well and we have the documents on it, so basically it
will be our standard:
The second Tc-99 disc has been well used:
Surface scratches led us to question the veracity of it's present activity. Sure enough,
it only gave a 150 CPM reading on the pancake compared to a 400 CPM reading from
the gently stored disc.
Careful measurements were taken, being sure that the deadtime of the tube was not an issue, and with those
readings and some math we decided to remark the second Tc-99 disc as being
0.00132 uCi today.
Now the Pm-147 gives a reading of 500 CPM at close contact. One Tc-99
disc gives 400 CPM while the other gives 150 CPM.
Together the two Tc-99 discs give nearly the exact same reading as the
Pm-147. So we can say the Pm-147 is equivalent to 0.0047 uCi Tc-99.
Observe the efficiency of a pancake probe to Tc-99 compared to
Pm-147:
Paul's chart above indicates about 12% eff. for Tc-99 compared to about
7% for Pm-147 based on maximum energy.
Ludlum's chart also indicates 12% for Tc-99 but based on the average
energy:
Hoffman's chart is probably more realistic, indicated the spread between
actual tests made with different pancake probes and based on average
energy:
Without a calibrated source to compare against, any reading of a known
energy beta emitter can only be accurate to a certain extent because of probe
inconsistencies.
We took apart a pancake probe that had a broken glass seal. Otherwise
the mica itself was undamaged. Careful measurements of the thickness of the mica
with and without the DAG coating are shown.
Intact 0.01 mm:
Weighing in at an incredible 0.034 g:
With DAG coating removed (clear as glass and
beyond the capability of the digital caliper- reads 0.000
mm):
We suspect the real reason the Hoffman chart shows such variability at a
given beta energy is the DAG coating. Here shown to have many voids and
variances:
At sea level the mica is slightly bowed inwards due to the reduced
pressure of the neon + chlorine gas inside. At higher altitudes, the mica
flexes, and at about 8000 feet of altitude or equivalent air pressure, the bow
reverses from concave to convex (Passenger airplanes are pressurized to 8000 ft
equiv).
Our GEO-210 probe recognizes that issue and provides a 1/16th inch gap
between the tube and the screen. This gap has saved many a probe going over the
Continental Divide @ 7245 ft.
One time while transporting a dozen or more instruments across the
Divide, only a brand new Ludlum popped ( Grrr...)
That incident prompted the invention of the GEO-310 also known as the
Pike's Peak Prospector:
This normal flexing happens to some extent everyday, even due to air pressure changes due to weather
fronts.
In time the DAG coating will flake off to some extent, changing the
relative window density:
OK - back to the beta efficiency charts. Here's an excerpt from an Eberline pancake probe's
paperwork:
That one says: Tc-99- (eMax 0.29 MeV) approximately 30% of 2 Pi emission
rate.
Let's address the Pi factor for a moment, it is
very important and often misunderstood or misused. Consider the spec of
radioactive material that makes up our test discs, it a tiny spec of
contamination on a surface. The radioactive isotope decays at a certain rate,
that radiation leaving the spec equally in all possible directions, think of it
a as a sphere or hollow ball with the spec in the center. Radioactive "activity"
is measured using the units Curie.
We use microCuries,
abbreviated uCi for our small sources, one
one-millionth of a Curie. Any source will have a disintegration rate of 2.22
million DPM (Disintegrations per Minute).
This radiation goes off in all directions,
forming a sphere The whole surface of this sphere represents the radiation flux
at that distance.
Naturally the only way we can intercept all locations on the surface of
the sphere at the same time, the source must be INSIDE the detector, or several
detectors can be placed to surround the source. Our method is far simpler than
that, using a single pancake probe, we can at best see 1/2 of that sphere, a
mathematical quantity referred tom as 2 Pi Radians, or 2 Pi. Relative placement
of a probe to a source is called its ?geometry".
It should be apparent that changing the geometry, that is, the distance,
angle of view and so on will change the final reading. Especially when
considering the "inverse squared law" which allows a X
2 change in distance to equal a X4 change in radiation
reading.
The perfect geometry would be a source placed far enough away to
illuminate the entire sensitive volume of the probe equally. This distance is
usually considered to be 10X the major dimension of the probe.
For a 2" pancake, that would be 20 inches.
Betas are attenuated in air at a rate that would give them a range of
only 10 to 12 feet per MeV of energy. A perfect setup
would use a strong source in a vacuum along weight the probe, not practical for
this experiment. We use a single pancake at close range to reduce air
attenuation.
Our measurements would correspond to a 2 Pi
geometry.
Be careful when reading beta efficiency specifications given by
manufacturers. Some are 4 Pi, some are 2 Pi, some are
charted as maximum energy (Emax) some as average
energy (Eav). As long as we recognize these facts, we
can compare different charts intelligently.
Step 2- Determine if the source is actually Pm-147:
Method: testing half value layer with absorbers, compare to a known
similar sample.
Careful measurements were taken of the Tc-99 source. Results in CPM were
recorded. Absorbers from the Spectrum Techniques were placed between the source
and detector until one was found that reduced the CPM reading by 50% This is the half-value-layer
for this particular beta energy. The absorber selected by experiment was
the B sample, 1 mil of aluminum sheet, 5.6 mg/cm^2.
Next careful CPM readings were taken of the bare Pm-147 source and
recorded. Then the B
Absorber was placed between the source and detector and the new CPM
reading recorded.
Comparing the to
values of CPM confirms that the beta energy of the source marked Pm-147 is
indeed of the expected range.
Step 3 - The experiment:
For our test, we will use Paul's chart as published in the RSO magazine
because it lists all the isotopes we are using:
Fortunately we are not concerned about absolute efficiency numbers,
rather the DIFFERENCE in efficiency of Tc-99 compared to Pm-147.
This brings us back to 12% and 7% respectively. Doing the simple math
results in a measured activity of 0.008342 uCi of
Pm-147 left.
So how old is a 75 uCi Pm-147 that is now
.008342 uCi?
The formula for decay= t = -(T1/2/0.693) *
ln(A/Ao)
where
-
A= final activity
Ao= initial
activity
t= decay time
0.693 = Lambda, the decay constant
Pm-147 =
2.62Y
Solving for t = 34.41 Y ago = Date of manufacture 5 Feb
1975
Believe me, using an online calculator to do the math or at least to
check the math is recommended:
http://www.radprocalculator.com/Decay.aspx
Have
fun,
Geo
George Dowell
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