or: Measuring Beta Backscatter from Elements of Differing
Atomic Number
Material list:
1) A beta Source- Sr-90 used here (note 1), Tl-204 works well too. Cs-137
and other mixed sources may be used for their beta component in this experiment.
If a mixed source such as Cs-137 (beta+gamma) is used, make sure to
include extra shielding, such as 1/4" Pb (lead).
2) A coin, a US Quarter or $1 Euro.
3) Beta Probe. Pancake probe
used here.
4) Counter/Scaler to take
accurate measurements over a time period. Ours is the SPECTECH
ST-350.
5) Reflector media. We have pure
samples of Al, Fe, Co, Ni, Cu, Ag, Sn, Pb, plus try plastic, glass, paper and
wood. You can use whatever you have around. All targets should have the same
dimensions. Ours are 1" X 1 1/4". Thickness
does not matter much for these tests.
NOTE: Where available, high resolution
picture links follow photos.
Lab manual:
A coin shield is taped behind the beta source so that the radiation only goes
away from the probe:
FIG 1
Then the source
is taped to the end of a wooden block, this becomes the jig which holds
everything stablewhile we change
reflectors.
The test jig is positioned
a short distance in front of the beta probe (pancake probe, thin end window, or
thin sidewall GM) in such a way that none of the radiation reaches the probe directly. Once the jig is assembled, take
a baseline background count with no reflector inserted. This count will be
subtracted from all further counts, to yield a net contribution of only the reflected/backscattered
betas.
FIG2
Set the scaler to run for
60 seconds, totaling the results when finished. A longer run time will be
statistically more significant.
Start by taking a baseline
background reading of the jig and source but no target. There is always a little
reflection from the jig itself,
and even your own body, so take an
accurate count and subtract this number from the individual reflector target
counts to yield a net CPM.
Gross CPM - Background CPM = Net CPM due to target
backscatter.
expressed
as: G^CPM - B^CPM = N^CPM
Next, one at a time, different reflector materials are
placed in the same carefully measured
position in front of the beta source.
Space it away about an inch.
Notice the space mark on the jig base.
What we are looking for is
the effect of different atomic number (Z) materials on a beta stream from the
source, only those
betas that are scattered
180 degrees or close to it are detected.
Our test results:
13 Al- 759 net
CPM
FIG 3
http://www.qsl.net/k/k0ff//Beta%20Backscatter/Aluminum%20Al.jpg
26 Fe-1038 net
CPM
FIG
4
27 Co-1056 net
CPM
FIG
5
28 Ni- 1055 net
CPM
FIG
6
29 Cu-1114 net
CPM
FIG
7
47 Ag- 1320 net
CPM
FIG
8
http://www.qsl.net/k/k0ff//Beta%20Backscatter/Silver%20Ag.jpg
50 Sn- 1320 net CPM
FIG 9
82 Pb- 1821 net
CPM
FIG
10
Conclusions:
Betas will bounce off
(backscatter) more as the Z of the reflector is increased. Elements that are
very close in Z number
show little difference form
one another, due to the randomness of the radioactive decay process and
measuring uncertainties.
Elements far apart in Z
number have markedly different reflective properties to the beta
particles.
Chart of
Data
The correlation coefficient (Pearson's r) =
0.99
Fig 11
Note: Above chart custom prepared
for this paper by Dr R J Prettyman and used with permission.
Note, a timed run was
performed with Pb in the jig and a beta shield consisting of a 1.8 mm aluminum
sheet covering the probe. This is done to verify it is betas we are
seeing reflected and not Bremsstrahlung or PIXE generated characteristic X-Rays.
These do of course exist with our setup but their
contribution is minuscule with this detector, and totally swamped put by the
betas.
Suggestions for further
study:
Once you jig is set up and
tested, try using other household materials as a target. Plastic, wood, glass
are just a few.
FIG 12
*(note 1): Gamma rays tend to be
emitted in discreet energies, as do alphas. Betas on the other hand are emitted
in a BAND of energies, from zero, to the MAX for that particular isotope
(source). Sr-90 has a 100% yield of a beta of 546 keV MAX, 195.8 Average. Even
though backscattered betas do lose some energy in the scattering process, the
probe used here is very sensitive to betas, even of relatively low
energies. when betas are bent via a magnetic filed, they do not lose
energy, but the degree of bending has a direct relationship to the energy of
each beta. Refer to article below "Bending Betas" for projects and
details.
Associated articles in this series by George Dowell:
PIXE- Element identification
using Particle induced X-Ray Emission.
Measuring Beta Efficiency of a
Pancake Probe - Using beta sources to test beta efficiency of
probes.
Bending Betas- Bending betas
using magnets. demonstrates the principles of Magnetic Beta
Spectroscopy.
Nuclear Radiation Lab
Experiments With Absorbers- Using calibrated absorbers to determine beta
energy, alpha, beta and gamma penetration.
Have fun
Geerge
Dowell
