MODEL LET-2 FAR WEST TECHNOLOGY 5
volts if the detector has an effective diameter of 1 micron. The signal from the preamplifier and through the linear
amplifier is presented to a pulse height analyzer. A small magnet is placed on the counter where "magnet" is
written to turn on the alpha source. The system gain is then adjusted to place the alpha peak in a convenient
channel in the pulse height analyzer. For example, at an effective diameter of 1 micron, the average LET of the
244Cm alpha particles in 81.72 keV/u and if the peak is placed near channel 82, then the analyzer display is
approximately 1 keV/u per channel.
After this preliminary work, an alpha spectrum is accumulated. Data should be accumulated for at least 10
minutes or more so that the position of the peak may be accurately determined. The alpha source is then turned
off by removing the source magnet making the counter ready for data taking. Data are usually accumulated at
several gain settings so that the limited dynamic range of the pulse height analyzer does not restrict the results.
For example, the first run might be from about 40 to 400 keV/u (for the 400 channel analyzer). The second from 6
to 60 keV/u, the third from 1 to 10 and the fourth from noise up to 3 keV/u. This scheme allows overlap between
the various segments so that they may be fitted together accurately. Obviously analyzer non-linearities must be
known, as well as the precise gain shifts used to select the spectrum segments. We have used a sliding pulse
generator to advantage in determining the analyzer non-linearities. A simple very stable mercury pulser can be
used to measure the overall gain at the different settings.
After the data are accumulated it is good practice to take a noise spectrum for the highest gain segment. This will
allow subtraction of the electronically produced noise and usually allows good data to be obtained down to about
0.5 keV/u. A second alpha spectrum should also be taken to establish detector drift characteristics, if any.
Usually the detector can be expected to drift less than 1 channel out of 100 in 8 hours. Drift rates greater than this
generally can be traced to gas leakage, either through pinholes or occasionally to the Quick-Connect. Since the
latter is a o-ring sealed device a bit of dirt may allow some gas leakage. This can be cured in most cases by
repeated operation of the Quick-Connect. The Quick-Connect leakage is not a problem if your detector is fitted
with a bellows valve. Temperature changes can also cause gain shifts.
VIII. DATA REDUCTION METHODS
Some methods of data reduction are discussed in References 6, 7, and 8. Invariably, computer aided processing
is required because of the large number of data points gathered. Smoothing of the data before processing may
also be helpful. Both linear and quadratic smoothing have been used with success.
If only certain segments of the data are needed initially a simple program can usually be written for one of the
many programmable calculators now on the market. Such a program must include a smoothing routine if it is to
be satisfactory.
IX. REFERENCES
1. F. R. Shonka, J. E. Rose and G. Failla, "Conducting Plastic Equivalent to tissue, Air and Polystyrene",
A/Conf. 15/p/753, Second United Nations International Conference on Peaceful Uses of Atomic Energy, June
1958.
2. F. R. Shonka, J. E. Rose, and G. Failla, Progr. Nucl. Energy Ser., XII, I, 160 (1958).
3. P. W. Benjamin, C. D. Kemshall and J. Redfern, "A High Resolution Spherical Proportional Counter", Nuclear
Instruments and Methods, 59, 77 (1968).
4. B. Day, "A Portable LET Spectrometry System for the Calibration of the Concorde Radiation Meter", AWRE
057/69, Avg 1909
5. D. Srdoc, "Experimental Technique of Measurement of Microscopic Energy Distribution in Irradiated Matter
Using Rossi Counters", Radiation Research, 43, 302 (1970).
6. H. H. Rossi and W. Rosenzweig, "A Device for the Measurement of Dose as a Function of Specific Ionization",
Radiology, 64, 404 (1955).
7. H. H. Rossi and W. Rosenzweig, "Measurements of Neutron Dose as a Function of Linear Energy Transfer",
Radiation Research, 2, 417 (1955).
