Gamma Photon Nuclear Radiation Detector with PIN Diode
PINRAD gamma detector project.
Maxim Integrated Application Notes 2236
As you know, PIN diode can be used for detection of gamma radiation. Maxim Integrated announced a good materials for the experimental building of such type detector. Here you can find my implementation, including oscilloscope screenshots and short video. The PCB installed into metal coffe can with 6.5mm audio socket. Metal box is required for light and EMI shielding. I added a transistor filter on the 15V VDD line for the PIN diode. This circuit cut VDD noise and improve output signal purity.
Because of small active surface of the diode, usually PIN detector has very weak reaction to the natural background. Actually low energy sources below 100keV was not detectable by the PIN diodes I have. But it can be used as a basic detector for radioactive isotopes, however not robust as a Geiger Tube. The circuit has two points where you can capture the signals: TP1 and comparator output Vout. Each pulse is about 5uS. The amplitude on TP1 depend on gamma photon energy. The amplitude of pulses is vary between 200mV-1000V. Otherwise, comparator Vout is 5V 5uS rectangular shape pulses that can be connected to micro-controller input for counting.
The upper module has signal mixer gates and cable driver. One 74HC14 works perfect for this goal.The digitalized buffered signal output from the can probe is 1uS - 5uS rectangular shape positive pulses.
The PCB's installed one above the second with interconnection wires. After that you need to solder it to the can cover as on the pictures above. Each module need to be tested separately. Remove D1 or D2, depend which module you need to test first and check with the scope you get signal output from each module. If there any problem, start to test signal output from TP1 point directly. Some diodes has very low sensitivity, at least 1 of the 5 pieces I have in stock didn't triggered pulses at all. So I recommend to purchase several pieces from different batches in advance.
The pulse counter circuit build with PIC16F628A microprocessor and 16x2 HD44780 LCD. The counter board is less sensitive to layout quality. Lithium charger IC MCP73831T can be replaced with any ready lipo charge module from eBay or from Sparkfun. TPS78330 can be replaced with other low-drop 3V regulator. To allow low VDD support I disabled brownout reset in firmware. But if your specific sample of PIC16F628A will not operate stable at low VDD 3V then you can increase VDD to 3.3V, or power the processor from battery voltage directly. If you remove TPS78330 from the circuit take note that LCD contrast will require modification.
R31 value need to be selected between 1.5K-5.6K depend on LCD contrast, try different values if your LCD is over-contrasted or non-visible. To feed the can probe I selected Pololu 4V-25V step-up adjustable module because it very efficient and miniature. The can probe can be connected to the counter board with 1 meters of audio cable. Because the current drawn of each PIN diode board is 10mA, select the cable with lowest DC resistance.
Download PIC16F628A firmware for "PINRAD" detector with PCB layout for DIY ver.1.00
The modules layout in PDF is mirrored and ready for print-out if you make DIY etching PCB's.
Because of PIN diodes variety of sensitivity there is no way for clear and precise dose rate calibration, like it already done in DIY Geiger kits with Geiger tubes. The only one parameter the PIC software is counting is CPM (counts per minute). My PIN diodes samples I got from Digikey is QSE773 and BPW34. Both types where non sensitive to low range of nuclear energy of 60keV and non sensitive to natural background. But it detect higher energy isotopes, as uranium, thorium and cesium. I'll post here later more detailed information about energy range sensitivity.
Do not expect that PINRAD detector will be robust as DIY Geiger with SBM-20 or pancake tube. This is education project and not efficient detector for low radiation level. I would say as long as it education project, it also emergency type detector that starts beeping only when radioactive level in air are dangerous, or when you apply isotope in front of PIN diode area.
The video of captured pulses on the scope. The base level of noises in the video is high because the video was taken before I added transistor filter on bias voltage of PIN diode: