Gamma Spectrometry


The Theremino_MCA while being completely Freeware and OpenSource is a true multi-channel-laboratory Analyzer.

More information:
– Electrical schematics and Assembly plans: www.theremino.com/technical/schematics
– Software: www.theremino.com/downloads/radioactivity
– Hardware, DIY and kits: www.theremino.com/contacts/producers
– Images and videos: www.theremino.com/video-and-images
– Article about Open Source Electronics: crafts-to-gamma-spectrometry-signal conditioning-of-

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Hardware for the Gamma spectrometry

The Theremino_PmtAdapter contains a feedback loop that can keeps the tension stable even in the presence of strong temperature fluctuations. In this way the calibration remains accurate over time and rows of isotopes do not move and do not expand.

ATTENTION: For optimal performance use pipes PMT wired as shown in the file PmtAdapters.pdf – PMT for low impedance tubes (with resistors 1 Mega or even from 560k) cannot operate with these adapters. To use them you should replace their resistors, as indicated.

This adapter can be used with the well known freeware software PRA (We thank Marek Dolleiser for having paved the way for this kind of analysis, its software PRA is a reference for many years and helped a lot) but only with the Theremino_MCA you can do filtering and deleting useful background to get maximum information within a reasonable time.

This file includes the design of PCB, le immagini e le simulazioni SPICE: PMT_Adapter_V3.1
This is version 3.2 con molti piccoli miglioramenti: PMT_Adapter_V3.2
This is version 3.3 con ulteriori miglioramenti: PMT_Adapter_V 3.3

Most salient features:
– Compact only 50 X 70 mm
– No initial thermal drift due to the feedback loop.
– Adjustable voltage from 500 to 1500 V
– Very low power consumption only 10 mA @ 5 V
– Very low ripple only 100 UV
– Protected against short circuit
– Maximum power output 100 MW
– Preamp circuit and pulse enlargement (from 3/5 uS to 100 uS to be read by a PC sound card)

Caratteristiche tecniche:
– Compact only 50 X 70 mm
– No initial thermal drift thanks to the feedback circuit.
– Adjustable voltage from 500 to 1500 V
– Very low consumption at 5 v only 10 mA
– Ultra-low ripple only 100 UV
– Protected against short circuit
– Max power output 100 MW
– Preamp circuit and widening of built-in impulses (carries impulses from 3-5 us to 100 us to be read by a PC sound card)

The simple and neat reduces manufacturing defects and makes them readily apparent.

In the following pictures you see the PMT in rehearsal.

The circuit diagram and an impulse to sample that shows the noise level of ’ power supply, Note that this is a low-energy pulse.

In the latest versions of PmtAdapter noise is less than 100uV. Practically the only noise due to sampling in 16 bit soundcard, as shown in the two following images.

The first picture shows only noise sound card, the second noise with a separate connected.

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The complete system

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The Pmt Adapter is not in production, You can build it but contains a number of special components, hard to find and expensive. So we recommend that you contact Lello, who knows how to find a cheap components and also print a number of PCB for friends: ufficiotecnico@spray3d.it

The theremino team only deals with research and does not sell hardware. The system is completely "Freeware", "Open Source", "Not for Profit" and "DIY", but there are manufacturers who can provide modules assembled and tested at a great price. One could hardly self-build them spending less. For a list of the producers read this page: www.theremino.com/contacts/producers


A hoof to the Hamamatsu PMT R6095 (and the like)

In this ZIP file the complete project Eagle and the GCode for the cutter: PMT_Socket

These pictures show how to adapt the connectors to printed and how is the plinth ended (click on images to enlarge them)

The capacitor might also be welded across (with two insulating sleeves on leads) and, to avoid short circuits with the outer aluminum tube, It is good to wrap the whole area from PMT to printed circuit board, with a sheet of plastic sheet..


An MCA system for Apple (iPhone and iPad)

By popular demand Alessio has designed a special version of PmtAdapter, usable with software available on iPhone and iPad. The software is called Geiger bot, ed è un riferimento per la comunità Apple.

Theremino system - iPhone_MCA

The circuit diagram is very similar to PmtAdapter for PC, but I added a battery (There is the ’ USB powered). The signal amplitude is reduced considerably, You can send all ’ mic input, that otherwise would saturate and would distort the pulse shape.

Theremino system - iPhone_MCA Am241 Theremino system - iPhone_MCA - Cs137

Here you can see the spectra obtained with Americium and cesium. Thanks to the wonderful display resolution “retina”, the words are so small, that do not disturb the vision of the beautiful black background.

We are light years away from a true MCA, line width “FWHM” (that is the most important parameter for an MCA) is overwhelming. Minor details of the spectrum are completely invisible. Here's the same Spectra produced by Theremino MCA:

Theremino MCA - Am241 spectrum Theremino MCA - Cs137 spectrum

With a Tablet 12 inches by 180 Euro (with Windows 10 and shipping included in price), you would have a portable instrument much more convenient and accurate. But the satisfaction of using an Apple system, It costs an exaggeration, is priceless!


Calibration and temperature

The Pmt Adapter and the photomultiplier consume only a few tens of milli Watts, that are insufficient to cause significant temperature variations. So you don't have to wait for a “warm up time” between the ignition and the measures. And there ’ is not even a progressive warming during very long measurements.

Though the crystals change with ambient temperature performance scintillators, as you can see in the image below:

Scintillation chrystals temperature response graph

Note that the response to temperature is not linear and that changes even slope from positive to negative, right in the area of normal temperature. Whereby an automatic correction would be inaccurate. It would take a correction table to be calibrated for each Crystal and this would be very complicated and ultimately unreliable. Much better make an adjustment with two markers before each measurement.

Check with the marker before each measurement is quick, accurate and very reliable. It is recommended to keep in place two small samples (for example, Caesium and Americium) at appropriate distances, so you always have two small rows of reference. Caesium is weaker and you hold it close enough to the probe while l ’ Americium hold about ten centimeters, or is enclosed in a capsule, to decrease its activity and keep him close to the probe.

The two lines should be the same height and small enough not to disturb the measurements. If you do not have to measure precisely the cesium and l ’ Americium, then the two markers can always stay in position. See their lines on the final chart (possibly commented) from the security that the chart is perfectly calibrated.


Build a well in lead

To use to measure the level of radioactivity and radioactive substances, natural background.

PozzettoMisura_G-Ray_MR02_eng

The measure consists of a base plate, plus some concentric cylinders, of various sizes and thicknesses. Among the components leaves something’ of slack, in order to facilitate the Assembly ’.

The material is actually brass lead, to distinguish it from the ’ probe grey aluminium, While the piece of inner cylinder (barely visible) is the unique ’ of plastic, with concentric ring in lead.

All the lead parts were made from sheet metal 1,5 mm thick, cut with a common Bill-sheet and shaped by hand, wrap around cylinders of aluminium/steel/plastic I had available (used only as “DIME”).

The base plate is made by folding several times a strip of sheet metal of equal width, getting a decent thickness.

Sheet bends easily around the straight edge of a table and flattens with hammer taps. The lead is very malleable.

All the pieces were wrapped with paper wide enough, so handle them does not come into direct contact with lead, that tends to get your hands dirty. Also why is good to calculate a little’ of slack between the various diameters of pieces, so then that paper coating doesn't break any insertion/extraction of cylinders all ’ same, that partially must fit one inside ’ more (as for the cylinder and top shirt)

To measure you follow this pattern:

  1. First arises on a table the base plate and the cylinder base;
  2. You insert the sample to be tested in the base tube, so that it touches the inside (lower) Dell ’ ring of the measuring Chamber: You can tuck under to it appropriate spacers;
  3. You put the measuring Chamber (plastic cylinder and lead supportive ring) so the below sample is centered on the bottom of ’ ring;
  4. He slips his shirt more than basic ’ inside the cylinder: It will support the plastic pipe and concentric ring of ’ will lead to the hole to allow subsequent insertion of the probe;
  5. You insert the probe : in my case straight draw in ’ ring of lead measuring nursery.

From the tests I've done this setup reduces background noise for almost 20 CPS to 3,1 CPS

Marco Russiani

Download this project, complete with more information and pictures:
Theremino System - Theremino_Pozzetto_di_Misura_ITA.pdf
Theremino System - Theremino_G-Ray_Test_Chamber_ENG.pdf

  1. Luca CAMPANA says:

    Thanks for the exhaustive clarifications.

    I will keep you informed of any developments

    Cordially

    Luca

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