Posted on by maxfomitchev

Kromek SPEAR CZT is a compact gamma detector with the useful energy range of 30 keV to about 300 keV, priced just under $2,000 US dollars – Fig. 1.

Kromek Spear CZT Gamma Detector
Fig. 1. Kromek SPEAR CZT gamma detector.

Although the manufacturer-declared energy range for SPEAR is 10 keV to 1 MeV, the small 5 x 5 x 5 mm2 crystal size results in the rapid reduction of the detection efficiency with energy. By the same token, the noise in the built-in preamplifier and the thickness and composition of the window severely limits detection below 30 keV.

The complete specifications of the SPEAR are shown on Fig. 2.

Fig. 2. Kromek SPEAR specifications adapted from the original PDF.

Kromek offers a turn-key USB gamma spectrometer GR1 CZT, which is is very similar to SPEAR, except for the larger 10 x 10 x 10 mm2 crystal size and USB connectivity. Unfortunately, high cost of GR1 (approximately $16,000) places this instrument out of reach of the vast majority of nuclear science enthusiasts.

SPEAR Spectrometer

Fortunately, with the help of the PulseCounter software it is possible to make a fully fledged spectrometer out of Kromek SPEAR using only a minimal set of hardware. In this experiment I have used an Ortec 456 high voltage power supply and an Ortec 490 amplifier mounted in a Berkley Nucleonics Portanim NIM rack – Fig. 3.

Fig. 3. Berkley Nucleonics Portanim rack with Ortec 456 power supply and Ortec 490 amplifier.

Strictly speaking the NIM rack and the Ortec 490 amplifier were not necessary, but it was convenient for me to use the Ortec 490’s 9-pin output to provide +12V power to SPEAR.

To complete the gamma spectrometer I have taken the following steps:

  • connected the SHV input of SPEAR to the Ortec 456 and set the bias voltage to 1000V;
  • connected the 9-pin Sub D connector of SPEAR to the Ortec 490 preamp power jack;
  • connected the signal output BNC of SPEAR to the PicoScope 4262 digital oscilloscope channel A via a 50-ohm impedance converter;
  • Setup the PulseCounter software to use 20 mV signal range, specified the sampling rate of 3 MHz and set the differential trigger to 0.1 mV (the signal noise level had to be quite low).

57Co And 133Ba Spectra

Figures 4-6 depict 57Co and 133Ba spectra captured using this setup.

Fig. 4. 57Co spectrum captured using Kromek SPEAR.
Fig. 5. 133Ba spectrum captured using Kromek SPEAR.
Fig. 6. Low-energy peaks of the 133Ba spectrum captured using Kromek SPEAR.

As expected, 356 and 383 keV 133Ba peaks are quite depressed compared to the 81 and 31 keV peaks (Fig. 5). In the same time zooming into the low energy peaks reveals significant level of detail about the peak structure (Fig. 6).

However, 57Co spectrum is somewhat disappointing due to the total lack of 6 and 14 keV lines (this is where the 30 keV cut off comes into play).

Conclusion and Notes

  1. Kromek SPEAR can be used with the PulseCounter software to capture high resolution gamma spectra. No additional hardware is required beyond the 1000V bias and 12V DC preamp power supply.
  2. Useful energy range is from 20 keV (due to the preamp cut-off) to 300 keV (due to the loss of efficiency).
  3. Extremely low-noise data acquisition system is required, e.g. PicoScope 4224 or PicoScope 4262 digital USB oscilloscope (useful signal magnitude without the amplifier is limited to 20 mV).
  4. A run of the mill NIM amplifier (such as Ortec 490) is not likely to produce a quality spectrum due to large amount of noise.
  5. If you want to amplify the SPEAR signal a low-noise amplifier (such as Kromek EV-550) is required, which may be pricey (e.g. EV-550 is just under $1,000 USD).