Posted on by maxfomitchev

I have setup an alpha spectroscopy test system using an Ortec Soloist alpha spectrometer (brochure) and an MCA-PRO as follows (Fig. 1):

  • I have installed an Ortec Soloist into a BNC Portanium AP-3 portable NIM crate to provide the +/-12 & +/-24V power to the spectrometer;
  • I have installed an Ortec A-series PIPS detector into the Ortec Solist vacuum chamber;
  • I have connected the Ortec Soloist VACUUM outlet to a Pfeiffer MVP-035 oil-free vacuum diaphragm pump (I do not recommend using oil-based mechanical pumps as oil vapor tends to contaminate samples);
  • I have connected the LIN AMP output of the Ortec Soloist to the MCA-PRO channel 1.
Ortec Soloist
Fig. 1. Ortec Soloist Alpha Spectrometer Setup.

Detector Setup

I have connected a digital multimeter to the HV test point in the back of the Soloist and used the DET BIAS ADJ potentiometer to set the detector bias to +50V.

Calibration

I have calibrated the Ortec Soloist using the built-in pulser by setting the pulser amplitude to 5 MeV using the PULSER dial on the front panel of the Soloist. Then I set the BIAS switch on the front panel of the Soloist to the PULSER position and connected the LIN AMP output of the Soloist to a digital oscilloscope and checked the test pulse amplitude, which was expected to be close to 5V. Because the pulse amplitude was a little off I used the AMP GAIN potentiometer located on the front panel of the Soloist to adjust the linear amplifier gain to get the pulse amplitude to match 5V.

I have repeated this calibration procedure several times using different pulser settings (e.g. 1, 5, and 9 MeV) to ensure that the 0 to 10V LIN AMP output range matched well the 0 to 10 MeV pulser test pulse amplitudes.

MCA-PRO Preset

Because the Ortec Soloist LIN AMP connector outputs positive pulses with pulse height corresponding to the alpha particle energy, I have configured MCA-PRO preset as follows:

  • Sampling rate: 5 MHz
  • Coupling: DC
  • Signal Range: 10 V
  • Signal Polarity: Positive
  • Pulse Shaping: None
  • MCA Baseline: 0
  • Threshold: 100 mV (100-300 mV, depending on the noise level)
  • Threshold Type: Absolute

Test Spectra

Using the setup described above I have obtained the following test spectra:

  • 210Po, Fig. 2 (dirty source);
  • 241Am, Fig. 3 (from a smoke detector);
  • Depleted uranium chunk, Fig. 4;
  • Thorium wire, Fig. 5.
  • 210Po, Fig. 6 (brand new clean source);
Fig. 2. Po-210 Alpha Spectrum (dirty source).
Fig. 3. Am-241 Alpha Spectrum (from a smoke detector).
Fig. 4. Depleted Uranium Chunk Alpha Spectrum.
Fig. 5. Thorium Wire Alpha Spectrum.
Fig. 6. Po-210 Alpha Spectrum (brand new clean source).

Conclusion

MCA-PRO paired with Ortec Soloist make a fine alpha spectroscopy system. However, the following factors must be kept in mind if one wishes to obtain precise reference spectra:

  1. Sample chamber must be evacuated below 20 Torr; alpha particles quickly loose energy due to collisions with gas therefore sufficient vacuum conditions are necessary for spectroscopy applications; on the other hand vacuum conditions can be relaxed for simple counting applications; counting at atmospheric pressure is possible when the detector is located at point blank wrt to the sample; spectroscopy should also be possible when sample is deposited directly onto the detector with no or minimal air gap between the sample and the detector.
  2. Samples must be carefully prepared by thin film deposition on a suitable, clean, flat, low-background substrate; the deposited samples may not be coated or contaminated with dust or fingerprints because any superficial contamination results in alpha particle scattering which would ultimately distort the spectrum by broadening the peaks and shifting the distribution towards lower energies; also, bulk sources do not produce good reference spectra because alpha particles undergo scattering as they exit the bulk of the sample, therefore the resulting spectrum will be affected by peak broadening and peak smear in the direction of lower energy.

Thus, the biggest challenge for obtaining theoretically-precise reference alpha spectra is sample preparation, which in most cases requires chemical deposition of thin sample films to be measured.

In this post I have demonstrated the difference between an old (dirty) and a new (clean) 210Po alpha source: the old source produces a broad spectrum where is the new source gives off a sharp 5.4 MeV line that perfectly matches the theoretical expectation.

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