Normally the recoil energies of emitting and absorbing nuclei prevent the absorption of gamma rays emitted by the same element. The energies of the emitted gamma rays differ from the energies needed for absorption by an average of twice the recoil energy. This effect can be overcome, however, by thermal motions of the emitting atoms (Doppler broadening), physical motion of the entire source, or the existence of a recoil energy less than the lowest phonon energy of the lattice. The Mössbauer effect refers to the absorption of gamma rays through the third method. This makes it possible to measure extraordinarily small energy differences between nuclear states, which is useful in diverse fields of research, including biology and solid state physics. The purpose of this experiment is to study the absorption spectra of several different samples.

• Radioactive source in vibrating housing
• User's guide for Model W200 Pulse Generator and Servo Amplifier
• Proportional gas counter
• Preamplifier
• Amplifier, Amplifier/Single Channel Analyzer (AMP/SCA)
• Oscilloscope
• Computer with Multi-Channel Scaler (MCS) and Canberra Genie 2000 software for gamma ray data acquisition and analysis

First set up the apparatus and determine that each component of the experimental setup is working properly.

Check to see that the radioactive source is in its holder. Turn on the hand-held Geiger counter and sweep the area with the counter to check the effectiveness of the internal lead shielding.

Turn on the gas detector by providing a high voltage of about 2800V to it. On an oscilloscope, monitor pulses from it after amplification with the AMP/SCA unit. Adjust the gain of the latter so that pulses are several volts in amplitude, to be on scale for the MCS card in the computer (which accepts 0 to 8V pulses). A coarse gain of 500 and fine gain of 7.5 seems to work well.

Connect the SSP (Start Sweep Pulse) rear output of the W200 module to the TRIG (trigger) rear input of the MCS card, and the CAP (Channel Advance Pulse) rear output of the W200 module to the ADV (advance) rear input of the MCS card. Do not touch the small screws labeled BW, Gain and Offset on the front of the W200 unit, as they have been carefully adjusted. You can use a Scale of 3 for the thumb wheel switch on the front of the W200 module.

In logging on to the data acquisition computer, use a username and password (e.g., guest and guest). If a password is not provided, you can appear to be logged in, but with insufficient privileges to be able to detect the data acquisition card in the computer. The data acquisition software can be started by selecting Start -> Programs -> Genie 2000 -> Gamma Acquisition & Analysis . To communicate with the data acquisition card, select File -> Open Datasource; click on Source: Detector, select MOSSB and click Open. You are now ready to start acquiring gamma ray counts (on the Y axis) as a function of time within a sweep cycle (on the X axis); this is a “Multi-Channel Scaler” (MCS) spectrum.  Explore the various software options to identify the spectral features  for absorption due to various Fe materials. Note that for the external timing reference provided by the CAP signal to work properly, the MCS card has to be set to recognize the external pulses: select MCA, Adjust, and set the "dwell range" to "external." Other parameters that seem to work well are: Disc. Mode: ROI, Scan window: 2%, Scan start: 0%, Scan end: 100%, and 1024 channels for the full MCS scale.

Now, if you start collecting spectrum data, you may see the numbers of counts rise very quickly, especially if the detector and AMP/SCA gains are set high. The great majority of the pulses are noise and uninteresting. You can set a threshold to reject the low noise pulses by defining a ROI (Region Of Interest). This is set in a counterintuitive way: graphically using the two cursors on the screen display, one can set a lower and an upper limit for the pulses accepted by the MCS card. For instance, if the system is set for 1024 channels full scale along the X-axis, setting the lower cursor to channel 102 would correspond to 1/10 of full scale; since full scale is an 8V pulse, this would set a threshold of 800mV for pulses accepted for counting. In practice, setting the low cursor to about 1/20 of full scale (about channel 50) and the high cursor to full scale (channel 1024) accepts pulses between 400mV and 8V, which seems to work well and rejects the low noise pulses. After defining the acceptance range graphically, the ROI range has to be set by clicking the "ROI set" button.

Note that if you save a spectrum using File -> Save As… , the saved file is in a proprietary format (.CNF) that can only be reopened with the Canberra software. To produce a simple ascii dump of the channel contents, follow the procedure: Analyze -> E. Reporting… -> Template Name: Datadmp.tpl , then Execute . The data file produced is in:

 C:\Genie2k\Repfiles\   

(look at the date and time of files there) and can be opened with any spreadsheet program. An Excel template that contains a macro and instructions for parsing the data can be found here.

Turn on the source vibrator, place an ⁵⁷Fe absorber in front of the photomultiplier and obtain a Mössbauer absorption spectrum. (Note: natural Fe is only about 2% ⁵⁷Fe. Enriched Fe has significantly more. Since only ⁵⁷Fe is Mössbauer-active, it takes much less time to record a spectrum from enriched-Fe sources.)

Obtaining a good spectrum takes several hours or even several days. You should explore as many different types of absorber as possible. Perhaps you could come up with new materials to try out. In addition, you might explore the temperature dependence of the Mössbauer spectrum.

When the ⁵⁷Co source becomes weak (which happens after a few years because its halflife is short), then it may require several hours (or more!) of counting in order to get a visible signal.

You might find the WMOSS Mössbauer analysis software useful in analyzing your spectra.

F. E. Fujita, U. Gonser, R. W. Grant, P. Gütlich, S. S. Hafner, and C. E. Johnson, Mössbauer Spectroscopy, ed. U. Gonser, Heidelberg, New York (1975).

G. Wertheim, Mössbauer Effect: Principles and Applications, Academic Press, New York (1964).

G. Wertheim, "Mössbauer Effect in Chemistry and Solid State Physics," Science 144, 253 (1964); see also Am. J. Phys. 31, 1 (1963).

N. N. Greenwood and T. C. Gibb, Mössbauer Spectroscopy, Chapman and Hall Ltd., London (1971).

R. L. Mössbauer, "Recoiless Nuclear Resonance Absorption of Gamma Radiation," Science 137, 731 (1962).

W. M. Reif, "Zero and High Field Mössbauer Spectroscopy Studies of the Magnetic Ordering Behavior of One, Two and Three Dimensional Systems", Chemical Mössbauer Spectroscopy, ed. R. H. Herbert, Plenum Press, New York (1984).

D. P. E. Dickson and R. J. Berry, Mössbauer Spectroscopy, Cambridge University Press, New York (1986).