Photomultiplier Tube USB adapter for gamma-ray spectroscopy.
This project is focused on make avaiable for common people like you and me, for a cheap price, an USB powered photomultiplier tube adapter. This makes possible to students, hobbyists, geeks and common people, to build a powerfull tool to test radioactivity of the surrounding enviroment and in general of the items around us. It permits also not only to have a quantitative measure of radioactivity, like with a Geiger counter, but also a qualitative measure like the professional laboratories.
The process to made a gamma-ray spectrometer at home usally was difficult and complex. First you must search for a capable high voltage power supply to power a photomultiplier tube then, modify an USB audio card to aquire the pulses. In the end assemble the photomultiplier with it's scintillation crystal to make a scintillation probe and install the proper multi channel analyzer software.
My USB PMT adapter makes the building of a gamma-ray spectrometer easier and cheaper than ever: there are integrated into a convenient 5x10cm pcb all the building blocks needed except the scintillation probe.
What is a gamma ray spectrometer and why build one?
Because the detection of gamma ray by a gamma-ray spectrometer is "a light year" more sensible and accurate than a simple Geiger counter. A gamma-ray spectrum also tells you what's exactly inside of the analyzed sample.
By measuring the spectrum of energies it is possible to distinguish radioactive isotopes and appreciate the relative abundance. Each isotope produces gamma rays with energy focused on one or more lines. The shape of the resulting graph is a kind of "signature" that is used to detect radioactive substances present inthe sample under test.
You'll need a gamma-ray spectrometer also for finding the smallest ammounts of radioactive polluants. It could be used to verify if your foods or items are contamined even by the smallest ammount of radioactive particles.
What is needed to make a working gamma spectrometer?
- A personal computer (not included)
- A multi channel analyzer software like Theremino MCA or PRA by GammaSpectacular that are freely avaiable to download online. Click on the blu text to go to the software's original website download page. (free to download)
- My USB PMT adapter of course (pledge it now!)
- A gamma-ray scintillation probe (you can buy it or better, make at home).
- You can obtain parts to make a complete scintillation probe from The Rad Lab. He it's a very reliable source of complete probes, PMT tubes from Philips and Hamamatsu, voltage divider kits and some scintillator materials too. For NaI(Ti) crystals, the first choice it to ask the producer Epic Crystals. They sells crystals that are good, brand new and cheap. By the way... you can also search on eBay!
- The USB PMT adapter needs an eclosure to do the best performance but it can do the job also without. I strongly encourage the use of an alluminium enclosure. This one is what I'm currently using for my personal spectrometer: Banggood alluminium enclosure
My circuit was developed during last two years. I've made several versions and I'm proposing you the best overal performance, the digital version.
V3, green pcb.
Please note that during this campaign some little things in the PCB are inproved. The voltage selector has changed from 4 to 6 pin and I've added an additional shielding to minimize RFI noise. The pictures refeer to the pre-kickstarter campaign prototype.
- It has digital control of high voltage.
- It's lowest output voltage is 500V with 64 discrete incremental steps of 12,5V each.
- Only positive output voltage output.
- Current capability for PMT divider with total resistance >= 80Mohm. In fact I've tested it also with a 40Mohm PMT divider but I recomand a minimum of 80Mohm. About linearity there's no problem at all with such big values for voltage divider because the system is optimized for maximum 500 counts per seconds to obtain maximum linearity. For more info about the design of a photomultiplier voltage divider you can read this file: voltage divider design
- This system uses an audio codec that digitalize pulses as audio signal. It's a CM108AH 48Khz sample rate @ 16bit.
- The preamplifier/pulse shaper output is a bipolar enlarged pulse with a 100uSec width. This is the optimal pulse shape that match the codec circuit. For this reason the system is optimized for best performances at 500 or maximum 1000 counts per second.
- Maximum voltage output is 1287,5V. I've added an HV boost jumper that boost the output voltage to maximum 1600v!
- Firmware could be updated via a jtag port with a simple USBasp programmer.
- There is a RGB led for audio card status, voltage status.
- Blue light is the audio codec, it is turned off when it's drivers are not installed on the pc. It's bliking when it's acquiring data and is turned stable on in stan-by.
- The green led is stable on when the internal converter/power supply is generating and regulating the selected output voltage.
- The red led turns on when the selected output voltage is not reached or there is an overload and the output voltage couldn't be reached. It turns on also if you switch from an higher voltage to a much lower one during operation; it takes some time (few seconds) to discharge output filter capacitors.
How it works?
- First connect your scintillation probe to the USB PMT adapter.
- Second connect with a usb cable the adapter with your PC.
- Select the needed output voltage to operate your choosen PMT tube by dip-switch.
- Run your favourite multichannel analyzer software and make your measure.
How to select the desired output voltage?
The dip switch is a 6bit selector. The most significant bit is on the left, marked as "1" and the less significant bit on the right marked as "6".
A table with 64 values of voltage/dip settings is too big to be published here but you can download it clicking here as Excel file.
If you wish to calculate how to set the dip switch with a calculator, you can follow this procedure: let'a assume that you wish to select 825V. Subtract the minimum 500V that are the all dip switches turned off, from 825V. It results 325. Divide the result by 12,5 and you'll obtain 26. Use the calculator in digital mode, input 26 and you'll see the conversion result in binary. Looking from the front switch the proper pins on the dip switch according to the calculator binary output.
A "1" is a dip switch pin down, a "0" is a dip switch pin up.
Why there is only a BNC connector for HV and not another one for the pulses from the probe? What is doing that USB connector?
The USB connects the audio codec inside the board to the computer. It samples the pulses and makes them available to the PC as audio signal. It is also the power source for the board. 5V 180mA maximum 60mA typical.
The BNC outputs HV to the scintillation probe and inputs the pulses. There is no need of another BNC connector for pulse input because the board has an internal load resistor with proper coupling on HV to extract pulses directly on-board. Take a look at this post, at the end of it there are some info about scintillation probe voltage divider https://madexp.com/2019/06/09/diy-scintillation-probe-the-easy-way/
Or directly a typical voltage divider for the R6095 from iRad inc.https://madexp.com/wp-content/uploads/2019/06/R6095-790x1024.jpg
This project have seen the light during winter 2017 and at the moment it have 2 years of uninterupted development. The idea to make an affordable gamma ray spectrometer was inspired by the Theremino PMT USB adapter. I've found the Theremino website for a totally casually. They'r project inspired me curiosity and I've started to study they'r design. First I've decided what I like from it to what I don't like from it and I've changed the designe. I've added a USB audio codec per example and used 99% of SMD components.
The first prototype worked... not so well! The second PCB revision corrected some errors.
Adding an alluminium enclosure improved it's stability. At this point I've reached the version V2. I've published it as Open-Source with the complete design freely avaiable on my website madexp.com.
The V3, the current revision has a lot of improvements: I've removed the old analog pwm HV converter and added in it's place a digital one. I've also changed connector type from SMA to BNC and improved stability and noise.
Why is a good idea to pledge my project and what did you get if you decide to pledge it?
I've a small electronics lab where I'm making research and prototyping. I'm focused into research and development of electronics that must be capable to give to common people, studentas and hobbyist, access to measures, tools and capabilityies that usually are restricted to professionals because of electronics costs. I've already concluded a campaign on kickstarter (196% founded) for an ultra compact 5v to 900V high voltage generator that is intended to be a very convenient all-in-one building block for Geiger counters. This at half of the price of the crappy oriental stuff that you can buy online. With that campaign I've rised enought money to buy tools and materials to make this USB PMT adapter project. If you help me with this new project who will know what marvelous project I'll publish here next year?
If you like to help me you can pledge for:
- madexp.com branded radiation hazard stickers
- USB PMT adapter V3 with three radiation hazard stickers plus vinil label to be applied over your enclosure
Risks and challenges
There are no risk into this project. It's already developed, tested and functional. I just wish to share it and use the founds to improve it and buy new tooling, new test equipment to make new even more complex and ambitious projects.Learn about accountability on Kickstarter
- (30 days)