2,5" x 2,5" NaI(Tl) gamma spectroscopy probe

DIY 2,5" x 2,5" NaI(Tl) gamma spectroscopy probe

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Here's a quite useful project for anyone is interested in studying radioisotopes, or just for experimenting with gamma spectroscopy. Since the very early tests, this probe revealed to be very sensitive to weak radioactive sources, allowing to build up a decent gamma spectrum in a matter of minutes.

The core of the system is a Russian 2,5 x 2,5 inches Sodium Iodide Thallium doped scintillator that can be found on eBay from many eastern sellers.
An Hamamatsu model R6233 photomultiplier fits perfectly the scintillation crystal size.

The housing is made by solid aluminium and it consists of four pieces only.
It has been designed to properly and precisely couple the Russian scintillation crystal to the PMT, ensuring the necessary magnetic shielding and light sealing.



Image by Manticorp - Own work, CC BY-SA 3.0,


A scintillator is a material that exhibits scintillation when excited by ionising radiation; the intensity of the scintillation is proportional to the incident gamma photon that has generated it.

In fact the light emitted by the crystal is so weak that requires to be amplified by a photomultiplier tube; the PMT ensures gain factors in the order of 10^5 times depending upon the supply voltage (up to 2.7x10^5 for R6233 @ 1000Vdc)


The scintillation probe requires to be supplied by a device that provides very precise high voltage, low ripple and low drift output. Once decoupled by the high voltage, the output signal is fed to a low noise high fidelity sound blaster capable to acquire and sample the probe output signal.

MCA (Multi-Channel Analyser)

A typical MCA has a configurable number of channels that can be selected based on the number of amplitudes that have to be analysed; this mode is so called "pulse-height analysis" (PHA).

The resulting graph plot will be an histogram of frequency (input pulses in CPS or CPM) Vs. pulse amplitude; the graph represents the energy distribution of the radioactive sample that is being analysed.


View all 12 components

View all 4 project logs

  • 1
    Step 1


    • General assembly drawing

    • Photomultiplier adapter

    • Probe top cover

    • PMT locking ring

    • Probe outer shell

  • 2
    Step 2

    Wiring instructions for HAMAMATSU R6233 PMT

    Courtesy of Tom Hall from iRad

  • 3
    Step 3



View all 3 instructions

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antiElectron wrote 02/05/2018 at 19:45 point

Thanks for your kind contribute. :)

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S. V. Paulauskas wrote 07/02/2017 at 22:49 point

I've managed to compile geant on the windows Linux subshell without issue. I've been looking for a fun project to try and play with geant. I'd be happy to help out. 

What type of daq are you using? I'm putting together a lab, and an looking for some cost effective solutions. 

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antiElectron wrote 07/03/2017 at 11:44 point

Thanks for offering your support; I will be happy to supply any information you will ned for your simulation with GEANT.

About the DAC, I'm using an external Creative Labs sound blaster connected over USB ( and I'm using it at the following resolution:

24 bps @ 192000 (sampling rate)

the MCA is actually configured with 6000 channels and the pitch channel is 0.02.

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S. V. Paulauskas wrote 07/08/2017 at 11:46 point

I suppose I could have *read* the documentation you previously provided to learn more about the DAQ.

I've got some skeleton code up on github ( 

Would you be able to provide clearer images for the dimensions? 

Did you use any reflective material in the detector can? Also, did you use any epoxy to seal the NaI in the can? NaI is hygroscopic and will pull moisture out of the air. This damages the crystal and will degrade the performance of the detector.

  Are you sure? yes | no

antiElectron wrote 07/16/2017 at 14:43 point

Hello, no problem. I've quickly checked your link to GitHub to the source code for the simulation with Geant; I will try to play with it as soon as I have time. Thanks for sharing it! :)

About the NaI(Tl) scintillator, I will prepare a 2D drawing with accurate outer dimensions.

In the meanwhile let me tell you something more about the scintillator.

The outer case is made by thin aluminum and it is open on the upper side so that it can be coupled with the PMT. The reflector material seems to be in form of powder (probably MgO or Al2O3) and coats the inner wall and the bottom of the aluminium case. Inside is housed the NaI(Tl) crystal that is closed by an optical window for the coupling to the PMT. The glass is sealed by a resin to avoid moisture absorption by the crystal that, as you told in your comment, is highly hygroscopic.

In my setup the aluminium housing that holds the scintillator is air tight by means of silicon o-rings; inside the housing I've left a small bags with silica gel that should absorb the residual humidity inside (just to be sure..).

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S. V. Paulauskas wrote 06/29/2017 at 21:51 point

Have you performed any simulations of the detector with something like GEANT4?

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antiElectron wrote 07/02/2017 at 14:37 point

No, but thanks for suggesting it. Unfortunately GEANT4 appears to be available as source code and needs to be compiled with Visual Studio; executable files are not available (.. Am I right?..)

Anyway I will try to install it as soon as I have time.

About the probe testing I've quickly measured the efficiency and it is less than 7% (typically 6.5% to 7%) measured on Cs137 sample @ 662keV

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antiElectron wrote 06/20/2017 at 11:50 point

Hello, I've used silicone optical grease.

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sparks.ron wrote 02/02/2018 at 08:13 point

where did you get the grease? I need a very small quantity for my detector.

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S. V. Paulauskas wrote 06/20/2017 at 10:22 point

What type of coupling did you use between the scintillator and PMT interface?

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