DIY Scintillation probe: 63x63mm NaI(Tl)

Some time ago I’ve won a 63×63 NaI(Tl) crystal for around 80€ at eBay. It is an SDN52 a special crystal designed to be heat and vibration resistant. Let’s build a probe with it and a 76mm photokatode XP2421/SQ PMT

63×63 Scintillator

First problem to solve was how to tightly couple this heavy crystal with this PMT? I’ve solved it with the help of my 3D printer: I’ve designed and printed a fitting with TPU that’s a kind of rubber filament material. STL zip file: gasket  password: madexp

And now let’s put all togheter.

The crystal
The assembly

Finally I’ve soldered in place the dynode chain voltage divider PCB.


After covering the tube with black tape I am ready to test it.

The result is a nice 7.5%-8% FWHM @ Cs137 peak

SI-31G geiger

This is the biggest geiger tube I have seen! I’ve bought one for 25€ at eBay.

Glass tube, thermostable. Cathode: tungsten powder.

  • Length of the plateau: not less than 200V
  • Slope of the plateau: 0,07%/V
  • Recommended working voltage: 900V
  • Dosimetric characteristics N = 0.1 imp / min · cm2 at 1 microR / s              Nmax = 2 · 104 Nf = 31
  • Service life: impulses 5·107
  • Range of working temperature: +10 / +250 °C
  • Dimensions: ∅27 x 400mm 92g



Lead castle

A lead castle, also called a lead cave or a lead housing, is a structure composed of lead to provide shielding against gamma radiation in a variety of applications in the nuclear industry and other activities which use ionizing radiation.

This is what Wikipedia says about lead castle. In fact, to make gamma-ray spectrometry of samples of very low activity you’ll need to use a lead castle. You can see the gamma rays emitted by your sample to be measured as signal and background radiation as noise,  using a lead castle, it will shield your sample from background radiation improving signal/noise ratio of your measure. I’ve built mine from a camping inox stove, a tin can and some lead scrap.

First thing to do is search for a good inox stove of proper shape, it will be the external container of the lead castle. Second thing is buy a tin can of proper size to be burried inside the castle. It will become the inner wall of the well.

Is a good idea to cover the external surface of this tin an with 1mm of copper foil. It will supress the xray generated by the interaction of gamma rays from the sample to be tested and lead shield.

You can melt some lead with a torch into a proper steel can and pour it to form the lead shield. First, do this outside! Lead is cancirogen so, better to handle it with gloves and in open air.

I’ve then poured a 16mm thick lead shield to form the bottom of the lead castle.

Then I’ve puttend iside the inner tin can blocking it in place with the help of a 10kg hammer positioned over it. Remember the hydrostatic lift force! Lead is ultra dense, the lifting force generated by the lead displacement caused by the little tin can must be neutralized via an heavy weight object positioned over it. Finaly complete the castle pouring molten lead into the walls. This is the final result.

It will take many hours to coold down. I’ve tested it’s effectiveness putting inside it a Cs137 calibration source and measuring it with my 40x40mm NaI(Tl) scintillation probe. I’ve made two measures, one from inside and one outside the castle, keeping constant the distance between the source and crystal. Result? Take a look at the following picture.


Matthias Doe sent me some pictures of how his made it’s lead castle. Take a look and thank you Matthias!

This is the final result
Posted in DIY

OST 40x40mm NaI(Tl)

I’ve finally tested my brand new 40x40mm NaI(Tl) crystal from OST making with it a new probe for my lab. This time I’ve tested epoxy glue for assembly. The dynode voltage divider is home made via a simple toner transfert/etching process.

First thing to do is to glue the crystal inside it’s aluminium housing with some epoxy glue. I’ve simply used a 46mm internal diameter, 50mm lengt aluminium pipe as coupling pipe. It’s external dimensions are 50mm and it will fit the inner diameter of the main probe pipe.

The crystal outer diameter is 45mm, the 1mm total clearance between coupling pipe and the crystal is filled with epoxy by placing the glue around the crystal body and sliding it into position. Next thing is to couple the crystal with the PMT and block in position with some tape.

Now some more epoxy….

And then I’ve slided the assembly inside the body pipe until it will block against the blue tape. As you can see the yellow and blue tape are simple a note of color and the blue one is used as sliding-stop reference mark.

After few minutes the glue will be hard enough to test the assembly. I’ve added a BNC terminated cable soldering the cable into the voltage divider pads, needed 1000V from my USB PMT Adapter and placed a piece of uraninite under it to take a spectra.

A very nice result!!!! Around 8% FWHM but this could be improved because the PMT was exposed to light during assembly and it’s noisy so it will take a day to rest. Today I’ve finally verified it with Cs137

7,8% FWHM but the software is not catching the correct peak position. I can expect a 0,5% FWHM error. This means 7,3% FWHM of the crystal. Not bad, not bad at all!!!

Trinitite HPGe spectrum

Some months ago I’ve bought a trinitite sand from United Nuclear. They’re website description say:

Early in the morning on July 16th, 1945, the first Atomic Bomb was detonated at the Trinity test site in the New Mexico desert. The nuclear explosion produced a blast equivalent of 18,000 tons of TNT and a ½ mile diameter fireball – with a temperature over 10 million degrees Fahrenheit, far hotter than the surface of the Sun. The intense heat melted the New Mexico desert sand into a light green, glass-like substance which was later named “Trinitite”. The resulting crater lined with Trinitite was buried for security reasons not long after the explosion.

In a couple of weeks I’ve received this vial full of Trinitite grains.

I’ve tested it under UV light but this material not present any fluorescence. The second test is obtain a gamma-ray spectrum from it in order to see if it’s a fake or not. I’ve used my 30×40 NaI(Tl).

The scintillation crytal have a center well made for vials. In this way low energy gamma rays are collected by the crystal at the highest efficency. After several hours of analysis the result spectrum shows clarely Americium and Cesium isotopes made by neutron activation during the bomb explosion. This sand is real nuclear melted sand!!!

The first left peak is Americium and the small peak at 609keV are Bismuth and Cesium isotopes. My crystal have good sensibility but an FWHM of 11% is not enought to extract any fine detail.

UPDATE 10 October 2019

I’ve made this test again with a 63x63mm NaI(Tl) scintillation probe enclosed into a 50kg componible lead shield. The result is amazing!!!

Americium, Europium, Cesium and Potassium isotopes from the neutron activation are quiet recognizable.

My friend Stanislav Prytuliak had made a test of a trinitite sample bought on eBay using the HPGe spectrometer at Karlsrhue university.

The following picture is Copyright by Stanislav Prytuliak (C) 2019 and couldn’t be reproduced. The following picture is published by his permission.

Final conclusion? The trinitite sample is not fake!

DIY scintillation probe: alluminium enclosures

How to make a scintillation probe with aluminium enclosure? Is not that hard if you have a lathe, a mill and a drill. I start assembling the scintillation crystal with the photomultiplier and measuring them. From standard aluminium pipe size, I choose one for the body and one, as reducer, to fit the scintillator inside the body pipe. From an alluminium round bar I made the ened cap.

Body pipe and crystal adapter. It fits inside the body and have a center hole where I put the crystal
This pic is from another probe… as you can see is easy to scale up my design. In this case the aluminium adapter fits a 36mm crystal into a 40mm pipe

Black silicon assembly
How the crystal + adapter fits the pipe

The crystal adapter is a piece of pipe shaped to fit inside the body pipe and have an inner hole where I put the crystal and photomultiplier. I use silicon black glue to gle everything together. It is strong but can be removed easily if I need to re-open the assembly.

Photomultiplier, crystal and front adapter
Silicon grease that will couple the crystal to the PMT front window


To assemble the PMT+crystal I’m using an optical coupling grease.

Blocking the assembly with some black electrical tape
Fixing crystal + PMT inside the adapter with black silicon glue

After silicon glue hardened I assemble the voltage divider board to the photomultiplier wires. The divider board is home made etching it’s circuit on a piece of FR4 or bakelite copper clad board.

Front assembly


Voltage divider added


Bakelite voltage divider board

Now that everything is ready, I close the probe adding the end-cap and soldering the voltage divider + and – at the BNC female connector on the ending cap.

Ending cap inner view
Ending cap BNC connector

Usually I anodize my alluminium parts. Using different kind of alluminium alloys for different probe parts results in different color of the anodized surface.

Hi-Zinc content into the alluminium alloy makes it darker than standard aluminium

And this is the final result

DIY scintillation probe: the easy way

This guide is under continuative update. 
updated 9 June 2019
updated 12 June 2019
updated 21 June 2019


Many backers of my USB PMT adapter project on Kickstarter asked me to write a simple guide that explains how to make a scintillation probe to be used with the USB PMT adapter for gamma spectroscopy. This guide is focused on the “dirty cheap” way to construct one at home without special tools.

I know two ways of making a probe:

  • Alluminium turned enclosure with internal teflon spacers. Internal mu-metal shielding and internal voltage divider.
  • Black tape and scissors “dirty cheap” probe.

The first type of probe is rugged, reliable and hard to make at home if you don’t have access to a lathe, a mill and have the skills needed to precise machining alluminium and plastics. This type of construction will be discussed in a new guide that I’ll write in few weeks. The second type that is treated here is easy, quick to make. It could be easily improved adding a proper aluminium enclosure.

How is made a scintillation probe and what I need to make one?


Example drawing of a scintillation probe


Scintillation probe with a well-type scintillation crystal


A typical scintillation probe consists of a light tight enclosure with, inside of it, a photomultiplier tube (PMT) that is coupled to a scintillation crystal. The voltage divider needed to feed the tube and extract pulses usually is placed on the back of the PMT tube. The tube could have wires soldered directly to the voltage divider PCB or some kind of socket. In this case the divider is connected to the socket that is connected to the PMT.

To make one you need:

On the right I’ve added a direct link to my favourite suppliers.

  • A scintillation crystal OST Photonics and Epic Scintillator
  • Some silicon optical coupling grease The Rad Lab
  • A photomultiplier tube The Rad Lab
  • A voltage divider for the PMT and it’s socket (if it needs one) The Rad Lab
  • A light tight enclosure that puts all pieces together. In this case… just some well tight black electrical tape.

PMT tubes example

The following pictures shows a typical photomultiplier tube usable for gamma-ray spectroscopy. I strongly recomand you you to buy tubes from a reliable source. My favourite tubes are from Hamamatsu and specifically the R6095

R6095 PMT from iRad – The rad lab
R6095 with it’s socket
Front optical window

Scintillation crystals

There are many type of scintillation crystals avaiable. The most common are:

  • LYSO    usable but have internal Lu176 gamma emission
  • NaI(Tl)    this is the type of crystal to buy
  • CsI / CsI(Tl) / CsI(Na)    low FWHM compared to NaI(Ti) hard to find
  • BGO    low FWHM compared to NaI(Tl) harder to find

For gamma-ray spectroscopy the preferable type is NaI(Tl). They can be found on eBay from cheap from ex CCCP countries.

The quality of your spectrometer expressed by the FWHM capacity of the system crystal-PMT-amplifier-ADC must be in the range of 8% or below. Small value of FWHM indicates better resolution capability of the spectrometer. Usually surplus soviet crystals have a variable FWHM because of they age and storing condition in the range 8-12%. A good crystal must be colorless with ultra -brite white internal walls. If not maybe they could be yellowish or stained and this means that they are trash.

Russians NaI(Tl) crystals. Usualy they are enclosed in a round machined steel enclosure.
On the left a CsI undoped crystal. On the right four LYSO crystals. They could be used but I prefer to use the LYSO as radioactive calibration source because it’s internal Lu176 gamma emission or for display in my collection :3
Bicron plastic scintillators. Because they low Z they are useless for gamma spectroscopy

Copling between the crystal and the PMT tube

The optical face of the PMT tube is where the scintillation crystal will be coupled. For this parpuse use some specific silicon optical coupling grease. It is very cheap and you’ll need a very modest quantity.

An important thing to note is the size of crystal and PMT. You must use a PMT with an optical windows bigger than the crystal. If you cannot you’ll lose some resolution. For example if the crystal is 43mm diameter and PMT optical window is 38mm you can couple they and expect some little resolution loss. If you try to couple to a 24mm PMT this coupling doesn’t work well and must be avoided.

A perfect coupling. PMT is 30mm diameter and crystal is 28mm internal diameter. It will works just great!
Another good coupling. A 10mm crystal on the optical window of a 30mm PMT
This coupling is BAD! A 30mm PMT on a 43mm crystal
An acceptable coupling. A 38mm PMT on a 43mm crystal. Is not a good coupling,I expect some 1% FWHM loss but it will work

Assemply the probe

So, you’ve find all the needed parts to assemble a probe? Good! Lets build it.

First, clean your desk, clean your PMT’s optical window and your crystal’s optical window too. You must remove all kind of grease from it like fingerprints with some alcohol. Put the crystal face up and spread over it’s optical end some silicon optical grease. Put the PMT otical window in contact to the crystal and spread the grease with little circular movements, this will remove bubbles of air. Don’t apply any pressure. The grease will make an optical coupling film between the crystal and the PMT. Now with some piece of black tape fix the PMT in position over the crystal.

Now countinue to wrap the black tape around the tube. No light must enter. Take care to tight the tape and to tight the coupling between the PMT and the crystal. I’ve closed the end of my tube with some black silicon glue. If your tube needs a socket you’ll need to enclose the complete assembly into a light tight enclosure. This because you cannot put black silicon glue under the socket.

Now, if you have some adesive mu-metal shielding sheet, you can cut it and wrap aroun your probe to add magnetic shielding to the tube. This is a good thing to do but, the tube will work also without it so, don’t worry about it at the moment, you can always add it! Connect your voltage divider circuit and your coax cable. Your probe is ready to be tested. My USB PMT adapter supply positive voltage from the central point of a BNC connector. I suggest to use a short (<=1mt) RG59 coaxial cable to connect the probe with the board. One end of the cable must have a BNC male connector, the other end, screen shield of the cable goest to the voltage divider – “minus” ground/cathode, the inner insulated conductor of the coaxial cable, feeds the voltage divider with high voltage. It goes to the + “plus” anode. If you use a mu-metal shield, connect it to – “minus”.

Notes about voltage divider

Photomultiplier dynodes need specific voltages and voltage ratio between each other. The voltage divider is specific for each photomultiplier. This guide explains how voltage divider is calculated. The Rad Lab sells PMT’s with voltage divider kit and instructions to build it. I strongly suggest to buy it from him and follow the assembly instruction. Here you can see the instructions provided for the R6095 and R9420


CsI undoped

I’ve bought this 10x10x30mm CsI undoped crystal scintillator from Moscow. It’s looking great! Let we see what Saint Gobain tells about this type of crystal scintillator material csi-pure-material-data-sheet_69770 :

“Cesium Iodide is a material with high γ-ray stopping power due to its relative high density and atomic number. Undoped CsI, being an intrinsic scintillator, has very different scintillation proper-ties from the more widely used CsI(Tl) or CsI(Na) activated by Tl or Na respectively.Undoped CsI is mainly used in physics experiments because of its combination of fast timing and relatively high density. Its scintillation is heavily quenched at room temperature, and cooling improves the light output.CsI is slightly hygroscopic”

Pure CsI crystal

I’ve tested it under UV light to see if it’s fluorescent but… no way! It doesn’t lights up under UV.

CsI under UV light

The Bequerel experiment

I always desired to reproduce some experiments of first pioneers of radioactivity.

Bequerel exposing some photographic plates to the radioactivity of some uranium rocks obtained some foggy image of the radioactive ore. This very simple experiment gived him the “spark” that let him discovery the radioactivity.

How to reproduce it? Simply! I’ve some 4×5″ photographic chassis and some sheet film from Foma czech analog film factory.

The Foma 100ISO film is what I’m using on my large photography analog cameras.

I’ve exposed this film for 24h to the radioactivity of uraninite, autunite and to the alpha rays of an Americium alpha emitter capsule and also added a piece of image intensifying film from an old x-ray casette to a third chassis and I’ve re-done the experiment hoping to see the screen effectiveness in inproving exposure of the sheet film.

Then, I’ve developed and scanned it.

From the left to the right: photographic sheet film, chassis with uraninite, chassis with autunite and Americium

The result confirm at 100% that the radioactivity from autunite and uraninite could expose the sheet film. The image intensify screen confirm it’s efficency too. I must punctualize that I’ve made two errors inserting or extracting sheet film from the chassis because I’ve exposed it to light for about 40% of the surface. Neverthless I’ve obtained nice results. The Americium alpha rays doesn’t reached the sheet film at all, blocked by the 1mm plastic enclosure and the 2mm inner space between chassis “volet” and fim.

Autunite and uraninite without image intensify screen
Uraninite and autunite with image intensify screen.

LYSO + ZnS(Ag) new order

Today I’ve made a new order from The Rad Lab. This time I’ve found on it’s ebay store a stock of 20 LYSO crystals to be used as gamma scintillators with my new SiPM photomultiplier. I haven’t wrote anything about it? Ok… I’ve bought from Mouser this AFBR-S4N44C013 Broadcomm SiPM silicon photomultiplier tube and it needs some little scintillator crystal.

Broadcomm 4x4mm SiPM

I’m planning to test it with LYSO from The Rad Lab. If it works well I’ll made a kickstarter campaign for an ultra small gamma

LYSO crystals under UV light

spectrometer based appon LYSO + SiPM with USB connection.

There’s no high voltage need by the SiPM! This means that I could design a very compact circuit. I’ve also bough an ZnS(Ag) scintillator disc to test my PMT and the new SiPM with it to try some alpha particle counting.

ZnS(Ag) image by The Rad Lab