Lab D4: The Mossbauer Effect
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In this experiment you can observe the Mossbauer effect, a very accurate (one part in 1017!) measurement of the energy of a nuclear gamma ray. This measurement is carried out by using motion of the source to sweep the gamma-ray energy via the relativistic redshift. A gas proportional counter is used to detect the gamma ray and a multi-channel scaler to record the data. |
Prerequisite experiment:
Lab B1: Ionizing Radiation Safety.
Lab B4: The Gamma-Ray Spectrometer.
References
Radiation Safety
Before doing this experiment, you should do the radiation-safety experiment, to familiarize yourself with the survey meter and the personnel dosimeter.
The 57Co source is an open (non-encapsulated) source. Do not put your fingers directly on the 57Co. Keep it in its lead pig when not in use.
Use the green survey meter (Technical Associates model PUG 1AB, with P-11 probe) to monitor radiation levels in the lab, and wear one of the XETEX model 415A personal dosimeters while in the lab. Record the dosimeter reading at the start of the lab period and at the end. Usual laboratory safety practices should be observed (e.g., no eating in the lab, wash your hands afterwards).
Experimental Procedures
This is a long experiment. The work can be divided into the following sections:
· Understanding the radiation from the cobalt-57 source.
· Setting up the gas proportional counter.
· Setting up the multi-channel scaler to detect Mossbauer absorption lines.
· Finding the lines and getting good data.
· Interpreting the spacing of the lines.
A. Radiation from Cobalt-57.
It is a good idea to have a look at the lines from cobalt-57 with the NaI gamma ray detector, before working with the more delicate gas proportional counter. Ideally you should have done the gamma-ray spectroscopy experiment as preparation for this experiment.
The gamma-ray spectrometer consists of the Tracerlab NaI[Tl] detector, a HV supply and an amplifier in a NIM crate, and the PCA computer-based pulse-height analyzer. Set up with these settings.
· High voltage to photomultiplier: +1100 V.
·
· PCA on memory group E1, gain 1024, display 1024.
Put on the cesium-137 source and adjust the amplifier gain to put its main peak 661. Warning: the count rate cannot be too high or the pulse-height sags. Check this by moving the source a bit away from the counter. The peak position should not change. Now the spectrometer is calibrated so that one channel corresponds to 1 keV of gamma-ray energy.
Now put on the cobalt-57 source. You should see the following peaks:
· Big peak near bin 26, too near threshold to use.
· Broad peak at bin 97.
· High peak at bin 134, with a shoulder at bin 151. We think that this is the 121 keV peak expected, just not at the exact right place.
Figure xx shows this spectrum, along with the cesium-137 peak.
You should try to interpret these peaks using the energy-level diagram for iron-57 given in the appendix. The resolution of the sodium iodide detector is not sufficient to resolve the 14-kev gamma ray and the 6-keV K X-ray expected. You might want to turn up the gain of the amplifier and look for them. The peak at bin 97 probably does not come from iron-57 at all, but is due to the K X-rays from the iodine in the detector. You can check their energy in the Table of Isotopes.
B. The Gas Proportional Counter.
The sodium-iodide detector produces pulses accurately proportional to the energy deposited in the crystal by gamma and X rays. However, its energy resolution is limited by statistical fluctuations on the number of photo-electrons detected. The gas proportional counter produces a signal from electrons ionized in a gas by the interaction of lower-energy gamma and X rays, with higher statistical accuracy. The resolution of the gas proportional counter which we will use is limited by the electronic noise from the pre-amplifier. The counter works a bit like a Geiger counter, having a wire at the center of a cylindrical gas-filled volume, maintained at a high positive voltage so that it attracts electrons produced in the gas by ionizing radiation.
A spectrum from the proportional counter as set up to start using the MCS is shown below.
Figure 3.
Spectrum from gas proportional counter, showing peaks from the iron
atomic K X-ray at 6 keV (bin 91), the iron-57
nuclear gamma ray at 14 keV (bin 181). The instrumental parameters for
this run were: HV, +1800 V; Canberra
816 amplifier with positive input, unipolar
output, fixed gain x16, variable gain x4.0 .
Appendix I. The Cobalt-57 radioactive decay.
Cobalt-57 decays to iron-57 by beta decay, with a half-life of about 270 days. The energy-level scheme for iron-57 is shown in Figure AI-1 below. The decay almost always (99.8% of the time) goes to the JP = 5/2- excited state of the iron-57 nucleus. And most of the time this state decays in two steps, giving gamma rays of 122 keV and 14 keV. The 14-keV gamma ray is the line used to observe Mossbauer absorption. In 11% of decays the state goes directly to the ground state, giving a 136 keV gamma ray.
In addition to these three gamma rays, the iron-57 atom will emit abundant Ka and Kb atomic X-rays, at about 6 keV. Thus the dominant features of the spectrum from a sodium-iodide detector should be clear peaks at 6, 14 and 122 keV, with a weak line at 136 keV.
Figure AI-1. Energy-level scheme for iron-57,
from Lederer, C.M., J.M. Hollander, and I. Perlman, Table of
Isotopes, Sixth Edition (1967), 191.
Figure 3.
Nuclear properties of iron-57, from , from Lederer,
C.M., J.M. Hollander, and I. Perlman, Table of Isotopes, Sixth Edition
(1967), 190.
Figure 4.
table I from Stevens, John G., John C. Travis and James R. DeVoe, Analytical Chemistry 44, 384 (1972).
Figure 6.
Velocity standards.
Source unknown. Figure 5.
Isomer shifts. Source
unknown.