Introduction

•Radioactive nuclei decay in numerous ways: emitting electrons, protons, neutrons, alpha particles, gamma rays, x-rays, or some combination thereof.

•During Internal Conversion nuclear de-excitation energy is transferred directly to an atomic electron which is then ejected from the atom. This is followed by a characteristic x-ray emission as the ‘hole’ left by the electron is filled. (pictured below on left)

Precise K measurement of 197Pt:

a test of internal conversion theoryMark Hernberg, University of Iowa

J.C. Hardy, N. Nica Cyclotron Institute, Texas A&M University

Source Preparation

In preparation for irradiation, a sample of 197Pt (0.7 mg) was mixed with aqua-regia and spread manually over a thin strip of Mylar tape to create a homogeneous film.

This source was irradiated by thermal neutron activation at the TRIGA nuclear reactor at Texas A&M University Nuclear Science Center.

196Pt(n, ) --> 197mPt

Data Collection

ResultsSpectrum Analysis

•This process competes with gamma ray emission. (pictured in green above)

•The Internal Conversion Coefficient (ICC) is the calculated ratio between internal conversion and -ray emission.

Why Study Internal Conversion?

Until recently ICC measurements rarely had an uncertainty under 1%. Furthermore, various theoretical calculations differed with experiment and each other by a few percent, and in some cases 10% or more.

One important uncertainty in current ICC theories is deciding the fate of the ‘hole’ left behind by the ejected electron: is it filled immediately, or does it stay empty throughout the conversion process? Recent experiments have also pointed to a possible unknown factor missing from both theories.

Precise ICC measurements (<1%) can provide a clear verdict on the correct approach and furthermore are useful for:•Nuclear decay schemes

-Spin and parity assignments, -Transition rates, -Branching ratios

•Detector calibration

•X and rays emitted from the source were measured with an HPGe detector

Relative photopeak efficiencies were calibrated to 0.15%

•17 spectra recorded ~3 hours to 9 days after activation.

197mPt nucleus decays isomerically, conserving its atomic and mass number, to a ground state of 197Pt. This nuclear de-excitation yields two distinct decay energies. The relative intensities of these decays are used to calculate the ICC.

Calculations

K = K-shell fluorescence yield, 0.959(4)*

NK, N = total number of Kx or -rays found by integration of spectra

K, g = known detector efficiency at peak energies

K

K

KK N

N

1

Counts

•The clear, distinct gamma ray peak shows no sign of interfering radiation. However, major impurities plague the X-ray region: by the 17th spectrum there are over 10 different instances of interfering radiation. Each impurities’ contribution to the peaks must be carefully analyzed and subtracted from the total peak area to ensure precise results.

13 24.4K

Conclusions

Though these results are very preliminary, the data clearly disagree with previous experimental results and are now consistent with theoretical calculations.

However, the data lacks the high precision shown in measurements previously acquired with the HPGe detector at the Cyclotron Institute. Inspection of the spectra show that this is due to the high levels of impurities in the source. An additional experiment with a new 197Pt source is planned to reduce the levels of interfering radiation.

*E. Schönfeld, H. Jaen, NIM A 369 (1996) 527.

197 Au

191m Hg

191m Hg

Acknowledgements

Thanks to Dr. John C. Hardy and Dr. Ninel Nica for their support and guidance during the project.

To the Texas A&M Cyclotron for giving access to a challenging and exciting research program.

And to the National Science Foundation for its continued support of the REU program.

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