If anyone is interested in SCA scalers I have several Eberline MS-2, MS-1 ?scaler / stabilized assay meters for sale along with some Portable Davidson 2048 MCA’s. I also have some PIPS detectors available if you’re building an alpha spec vacuum chamber.

Dud KK7IF

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This link is a good one to read to get your (our) feet wet:?



He explains how to use a silicon sensor, how to bias it and what to do with the pulses it gives.
The route he chose is a direct one, using the CREMET modules linked in another thread here.?

The stages are relatively simple compared to say, a Gamma Ray Scintillometer.
Silicon diode is reverse-biased with a low DCV, protected from light because after all it is a photodiode, pulses are preamplified by a simple charged coupled amplifier (usually a FET), further modified by a shaping amplifier, then sent to a pulse height analyzer (PHA) of your choice.

The above article uses a Theremino MCA, but it could be a single channel analyzer (SCA), a regular commercial Multichannel Analyzer (MCA), or some other
system, such as an ordinary oscilloscope to vies the pulses.

SCA's come in all shapes and sizes, some are internally preset as to which channel, they will count, while others are adjustable externally with a 10-turn mechanical counter, where each division is a certain number of keV. A few SCA's are set up with the normal WINDOW/THRESHOLD adjustments but include a range switch as well. With an adjustable SCA, you can manually scan the pulse train to note counts in any of the MeV areas being produced by the sample. Peter has already done this last test with the sensor/preamp board from his Alpha-6.

A WINDOW/THRESHOLD setting is done like this:

THRESHOLD (also called lower-limit-discriminator or LLD) is set to provide the lower end where nothing lower than that is detected
WINDOW (also called ULD for upper-level-discriminator) set the upper end where nothing higher than that is counted. This control effectively sets the width of the window above the LLD which anything inside is detected and counter. The WINDOW automatically tracks with the THRESHOLD setting, so moving the latter will change both the upper and lower, keeping the width of the window that same always.

It should be obvious that if the LLD dial has 100 divisions, and each division is worth say100keV, then moving it until a peak occurs will give a good idea of the energy of that peak by reading the dial.

Truly accurate FWHM measurements are made on Gamma Spec. probes that way in the Home Lab.


Today's linked schematic is of the front end of another Eberline CAM, the Alpha-3 Air Monitor. Like the Eberline A-6 CAM, the front end contains the preamp and shaping amp right at the detector, sending those pulses back to an analyzer board elsewhere in the device. There is a great write-up about all these stages and steps in the A-3 CAM manual, also linked below.

I'm fortunate to have 2 of these A-3's in the Home Lab, with sensors.

Geo