Introduction: Multi Channel Analyzer MCA With Gamma Spectroscopy NaI(Tl) Detector

About: DIY Master


Welcome to all who are interested in hobby Gamma Spectroscopy. In this short article I just want to share my work-log process of creating homemade DIY Gamma Spectroscopy detector with MCA. It's not a guide, I share only photos of the process.

When I started the project, I decided to make portable battery operated device with good linearity and FWHM% resolution less than 8%. The circuit evolves high voltage for a photomultiplier tube, it has analog electronics to process pulse shape and it has digital electronics to count pulses and analyze spectra.

Step 1: Making NaI(Tl) Detector

The detector build with R9420 Hamamtsu photomultiplier tube and 30x40mm NaI(Tl) scintillation crystal. The crystal is optically coupled to the tube photo-cathode window. Tube is covered with several layers of electrical tape to prevent any external light photons from entering into photo-cathode. When a gamma ray strikes the crystal it produce micro light flash that intended to be detected by PMT tube. The intensity of the light flash consist information about energy of the gamma ray.

To drive the PMT tube we need high voltage. I created miniature and stabilized step-up 5V to 1000V converter. When you deal with gamma spectroscopy you need strictly regulated high voltage with good temperature compensation and long term stability. Modern electronic components allows to create this design.

The driver also includes voltage divider for dynodes and pulse-processing charge sensitive amplifier installed directly on anode wire. This compact design has low noise signal and helps to avoid ground loops.

The enclosure made with aluminum tube on home lathe machine. I'm not professional CNC, everything is done with manual work.

Under the cover (not shown on the photos) I installed additional small board with LiPO battery, charger and LED indicator. The detector power up automatically when cable is connected. Charging of the battery can be done with same cable and any 5V adapter.

You can see typical pulse shape scope screen from the detector. As is, it can be used with any computer based MCA software, for example PRA, Theremino or BecqMonitor2011. These software uses audio cart to analyze the signal.

After 2 or 3 evenings I spent on detector adjustments to find optimal high voltage and amplifier settings it ends up with pretty good linearity and ~7.30% FWHM% on 662keV

For the detector test I used freeware BecqMonitor2011 with 24bit audio adapter.

Step 2: Making Portable MCA

Because I planned to use my detector as a portable device, I made Multi Channel Analyzer that can capture signal and save spectra to uSD cart in CSV format.

I used MHH-95A enclosure and created PCB design of my MCA that fit this enclosure. The MCA has 8-bit PIC18 microprocessor with 10-bit ADC 1024 channels.

128x64 display show only partial information of spectra. Full 1024 bins data is saved to SD cart and can be opened later by BecqMonitor2011.

The MCA electronics powered by 2xAA batteries. It has 2 buttons to control the software and one button for On/Off.

Step 3: Results

The whole setup can detect gamma energy in range of 20keV-3000keV, has good linearity and ~7.30% FWHM% at 662keV.

The first spectra is 1 hour of Cs-137 log.scale. You also can see Ka-40 at 1460keV

Second spectra is antique radium watch Ra-226 linear scale 30 minutes

Third spectra is antique radium watch Ra-226 log. scale 30 minutes

Fourth spectra is thoriated lantern mantle Th-232 log. scale 30 minutes

Hope this article can bring inspiration for your next build!

Step 4: Conclusions and Cost

The project is NOT cheap. I don't have exact cost summary for every part I used in this project, but the most expensive are:

1. NaI(Tl) crystal. I purchased this sample new for about $200. It expensive mostly because it has guaranteed resolution and it manufactured in nowadays. Old stock crystals are problematic in my experience.

2. R9420 Photomultiplier Tube. $60 The PMT tube I used is not new, but in good condition from trusted supplier.

3. Enclosure manufacturing. Even I do it by myself it has cost and need a lot of time. The materials in small quantity I buy are expensive, for example tube, aluminum rod and plastic can cost you about $100 including shipping, you also need to add machining costs on tools, inserts etc.

4. Electronics prototyping and PCB manufacturing. The cost is high - $$$$, I even cannot count the total hours, days and months I spent on this subject. I addition I try to avoid low cost ebay-ali electronic components. The MCA microprocessor software wrote by me too. It took for me too many resources and time, as a self-employed maker and student I choose not to share my source files because it will never cover my costs indeed, sorry. But if you are creative and open to cooperation, you can write me a proposal for business cooperation.

5. All other parts around like cables, jacks, batteries, materials, glues, tapes and etc are about $100, yessmall things makes difference here...

The conclusions: In my opinion the project has great performance. I can analyze food, mushrooms, berries, find radon daughters in rain water, test concrete materials or minerals for radioactive isotopes in range of gamma energy 20keV-3000keV. Even with all high cost as a DIY project, it still very cheap if you compare it with professional laboratory grade gamma spectrometers. Most common and dangerous gamma isotopes can be easily detected by the device.

Step 5: Project Videos