Gamma
Ray Spectroscopy of Lutetium Metal Element
By George Dowell
Our test sample is 1 gram of
the natural rare-earth metallic element Lutetium (Lu - one of the Lanthanide
series), consisting of small turnings left over from some industrial process of
machining. Lutetium is also known as Cassiopeium (von
Welsbach).
Very rare and expensive, natural Lutetium contains 97.4% Lu-175, the stable
isotope, with 2.59% a very special isotope, Lu-176, which is mildly
radioactive. Lu-176 has a half-life of 37.6 Billion Years. Being totally
natural, there is no license required to own a sample (Note 1), however in the
pure metal form it should not be handled or ingested (Note 2).
The Spectrum Techniques UCS-20 MCA, with a 2" X 2" NaI(Tl) Well detector was set for 3600 second run.
Looking at the four distinct peaks we find first on the left a broad peak,
actually a merged set of individual X-Rays caused by Beta Particles exciting
the inner electron shell of the Lutetium atoms themselves.
Called "Characteristic X-Ray Fluorescence", the peaks are of very
specific energy levels in nature.
The Ka1 and Ka2 energies are 54 and 61 keV
respectively, as shown on the attached XRF Periodic Table of the Elements
(Fig.8)(Note 3).
Next there appears a peak at 202 and another at 307. These are Gamma Ray peaks,
with a remarkably high percentage of coincidence (emitting simultaneously). As
a result of this coincidence another peak, called the SUM PEAK clearly appears
at the sum of 202 + 307 = 509 keV.
Fig.1
The DECAY SCHEME of Natural Lu-176.
Fig.2
The Home Rad Lab with
Spectrum Techniques UCS-20 MCA and ST-150 Nuclear Lab Station
Fig.3
SPECTECH
UCS-20, Various shielded probes and the SPECTECH
RAS-20 Calibrated Absorber Set that we
will use in many experiments in this series. For this
Gamma Spectrum test we use the 2" X 2" NaI(Tl)
Well detector in the large white colored lead shield.
Fig.4
Our
sample of Natural Lutetium metal
turnings as received.
Our sample was
transferred into an unbreakable test tube for measurement and permanent
storage.
Using the Specific Activity formula we come up with 52 Bq
from this 1 gram sample.
1g X 2.59% = 25.9 mg of Lu-176
Lu-176 Activity is 2kBq/g or 2 Bq
per mg
25.9 X 2 = 52 Bq/g of nat
Lu.
Fig.
5
Natural
Lutetium sample weighed and prepared in a test tube before inserting
into the probe for testing.
The Spectrum Techniques
UCS-20 MCA, with a 2" X 2" NaI(Tl) Well detector was set for
3600 second run.
Looking at the four distinct peaks we find first on the left a broad peak,
actually a merged set of individual X-Rays caused by Beta Particles exciting
the inner electron shell of the Lutetium atoms themselves.
Called "Characteristic X-Ray Fluorescence", the peaks are of very
specific energy levels in nature.
The Ka1 and Ka2 energies are 54 and 61 keV
respectively, as shown on the attached XRF Periodic Table of the Elements
(Fig.8)(Note 3).
Next there appears a peak at 202 and another at 307. These are Gamma Ray peaks,
with a remarkably high percentage of coincidence (emitting simultaneously). As
a result of this coincidence another peak, called the SUM PEAK clearly appears
at the sum of 202 + 307 = 509 keV.
Fig.6
Lutetium
Gamma Spectrum after a 3600 second run, 3600 second "Background
Stripping" and "3 Point
Smoothing.
Using a natural isotope to provide a stable, rather precise 3 point calibration
has some distinct advantages.
Fig.7
Using the 3 distinct peaks from Lu-176 as a 3 point
calibration standard.
Just as a comparison we
tested the Natural Lutetium in the ST-150 Nuclear Lab Station which uses a thin
mica window GM probe.
Long run times were required
to detect the very faint radioactivity present. Various calibrated absorbers
are inserted between the source and probe to block Beta Particles yet pass the
Gamma radiation.
Fig.8
Testing the natural Lutetium under a thin mica window probe
in the SPECTECH ST-150 Nuclear Lab Station scaler.
Fig.9
The Periodic Table of the Elements, XRF Energies.
This segment deals with
several radiological phenomena:
A) Beta minus decay @589 keV Max
B) X-Rays caused by energetic Betas
C) Gamma Rays (202 and 307 keV)
D) Sum Peaks (202 +307 = 509)
Keywords:
Spectrum Techniques; Lutetium metal; Beta Decay; Characteristic X-Rays;
Isotope; Sum Peaks
Next time we will use a
SPECTECH Cs-137 source and separate the Gammas and X-Rays from the Beta
Particles,
then
perform some interesting tests using just the Betas.
Notes
(1) NRPB-R306 "100 Bq/g Exempt Activity
concentration, 1 MBq Exempt Quantity"
(2) MSDS: http://www.espi-metals.com/msds's/lutetium.htm
(#) Link to entire XRF
Chart: http://www.qsl.net/k/k0ff//Lu-176/X-ray%20Table.pdf
Have fun and thanks,
George Dowell