Gamma Ray Spectroscopy of Lutetium Metal Element

Transcription

1 Gamma Ray Spectroscopy of Lutetium Metal Element A Tutorial 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 turning 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 = 509 kev.

2 Fig.1 The Home Rad Lab with Spectrum Techniques UCS-20 MCA and ST-150 Nuclear Lab Station

3 Fig.2 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.

4 Fig.3 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.

5 Fig. 4 Natural Lutetium sample weighed and prepared in a test tube before inserting into the probe for testing.

6 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 = 509 kev. Fig.5 Lutetium Gamma Spectrum after a 3600 second run, 3600 second "Background Stripping" and "3 Point Smoothing.

7 Using a natural isotope to provide a stable, rather precise 3 point calibration has some distinct advantages. Fig.6 Using the 3 distinct peaks from Lu-176 as a 3 point calibration standard.

8 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.7 Testing the natural Lutetium under a thin mica window probe in the SPECTECH ST-150 Nuclear Lab Station scaler.

9 Fig.8 The Periodic Table of the Elements, XRF Energies. This segment deals with several radiological phenomena: A) Beta minus kev Max B) X-Rays caused by energetic Betas C) Gamma Rays (202 and 307 kev) D) Sum Peaks ( = 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.

10 Notes (1) NRPB-R306 "100 Bq/g Exempt Activity concentration, 1 MBq Exempt Quantity" (2) MSDS: (#) Link to entire XRF Chart: Have fun and thanks, George Dowell