Russian 2.5" x 2.5" CsI(Tl) Scintillator PC-based gamma spectrometer assembly 分光計

 Russian 2.5" x 2.5" CsI(Tl) Scintillator PC-based gamma spectrometer assembly

Date of production 2012

 Gamma Scintillation Detector  assembly for Amateur Gamma Spectrometry

Gamma Scintillation Detector Monoblock is based on large Caesium Iodide monocrystal (63 mm x 63 mm CsI(Tl)) coupled with 3” PMT, special low noise preamp, regulated High Voltage source and USB audio codec from Texas Instruments.

Caesium Iodide crystal (63 mm x 63 mm) coupled with 3’’ PMT are organizing hermetic, non-disassemble and thermo&vibro robust  construction and it came from Russian  military surplus warehouse.

Dimensions:

height is 260 mm,

diameter is 85 mm;

weight is 1 Kilogram

What you need to make it to work? Just plug the USB cable jacks into PC and monoblock. Then the operating system will independently find and will establish the Codec driver as an Human Interface Device (audio device) onto your system.  Ready to go! Compatible with Microsoft Windows XP, Vista, Windows 7. No special driver needed. PRA and Fitzpeaks software ready.

Detector assembly has a High Voltage trimmer and you can  vary PMT gain and position of basic line (Cs137 source, not included) in spectrum scale. High Voltage varies by trimmer from  900 up to 1600 Volts.

Large crystal dimensions makes detector very effective for wide range of gamma rays. Low noise preamplifier makes  it very sensitive in the field of weak energy spectrum (see Picture – Americium 241). Energy resolution of detector  is a complex dependent parameter. For a CsI(Tl) crystals with the sizes of 63x63 mm energy resolution usually not better than 10 percent, based on energy line 662 KeV (by Cs137).  See picture  – energy resolution based on Cs137 662 KeV line.

Please, see the spectra of most common isotopes. Activity of each – 1 uCi, distance between source and detector – 8 cm (3”).

 Technical notes and general tips.

1. Construction overview.

Scintillation detector assembly consists of 63*63 mm (2.5”*2.5”) CsI(Tl) monocrystal, FEU-139 photomultiplier tube, low-noise regulated adjustable power supply, low-noise preamplifier and ADC with USB interface.

63*63 mm CsI(Tl) detector is effective for gamma-spectrometry in more than 20 Kev – 3 Mev energy range. Typical energy resolution is slightly better than 10% for 662 Kev peak of Cs137 source. Light output of CsI(Tl) crystals depends on temperature, so after temperature change calibration is recommended. It is not recommended to change temperature of the detector faster than 1°Cminute.

Scintillation crystal is optically matched with 12-stage photomultiplier tube FEU-139. Voltage divider for photomultiplier is made as separate PCB unit. Anode of photomultiplier is separated from resistor divider for noise reduction. Photomultiplier is highly sensitive to magnetic fields. In most cases magnetic shielding of the detector assembly is recommended.

Photomultiplier power supply circuit is PWM type, output voltage is adjustable in 900-1600V range. Optimum voltage for FEU-139 photomultiplier is 1200 - 1300V and can be easily adjusted with small screwdriver. Whole voltage range corresponds to 10 revolutions of the trimming resistor for better accuracy. All power supply circuitry is hermetically sealed with silicone compound and shielded for emission reduction. Output high voltage is additionally filtered. Power consumption from USB port depends on actual voltage and less than 100 mA at maximum voltage.

Low-noise preamplifier performs signal conditioning before ADC. Total noise equivalent of photomultiplier and preamplifier is less than 5 Kev, so weak Americium-241 X-ray peaks of 13 and 17 Kev can be separated on spectrogram.

ADC is 16 bit and 44100 Hz sampling rate with plug and play compatibility (Windows XP, Vista, Windows 7), no additional software drivers are needed. Operating system will treat it as external USB audio device (HID class).

2. Sensitivity and load.

63*63mm CsI(Tl) scintillation detector have very high sensitivity: around 160 000 pulses per minute to microsievert per hour. It is normal that in typical gamma radiation background (0.1 uSv/h) you will see 250-300 counts per second, or – 15 000 – 18000 counts per minute.

ADC sampling rate is limited to 44100 Hz, so more than 1000 counts per second load is not recommended.

3. Software and technical tips.

Freeware PRA program is recommended for work with the detector assembly. It can be downloaded from author's web page at http://www.physics.usyd.edu.au/~marek/pra/index.html . FritzPeaks also works well.

Switching OFF of absolutely all sound effects for USB sound input device is strictly required. Some Windows drivers sets ADC gain to maximum possible. It strongly reduces actual dynamic range of the detector. If it happened you need to set input signal gain to 5-7% instead of 100%, high voltage adjustment is not required.

You can see initial setup and calibration (in Russian, with English subtitles) at http://www.youtube.com/watch?v=cvIVvM9n5ts. Optimum settings for your equipment depends on computer and operating system and may be different.

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