Theory of Radiation Monitoring

This topic is actually a response to the topic Continuous monitoring of radiation background in Moscow . I hope he helps those who wish to organize their own monitoring.

The fact is that, by occupation, I monitor the radiation situation. Initially, gamma-ray detectors were installed as additional detectors at two neutron monitor stations . However, the data obtained from them turned out to be quite interesting scientifically, so we began to slowly develop this topic.
In general, two of the simple and relatively affordable methods of detecting radiation can be distinguished:
1. using Geiger counters
2. using scintillators

1. Geiger Counters

You can read about the Geiger counter device on Wikipedia, however novice dosimetrists usually miss two points.

1.1. Physical parameters of the counter.

Geiger counters are very different. To monitor radiation, first of all, you need to pay attention to the wall thickness and background count.
The wall thickness depends on the brand of the meter and determines its effectiveness when registering radiation. In this regard, the SBM-20 and SBM-21 meters, which are easily mined from Soviet dosimeters, are most suitable for our purposes. Also often come across counters like STS-5. They have too thin walls, but they can be wrapped with ~ 0.5mm foil and a decent detector will be obtained. Otherwise, he will count secondary electrons from the environment.
Background (natural) accountThe detector should be low enough. Usually 25-50 pulses per minute. A higher score leads to poor statistics and difficulty in registering small variations. For example, with STS-6, the background count is already 110 imp / min. However, it is possible to deal with this, including several counters in parallel, to obtain either a total score at the output or averaging data from several registration channels.

1.2. Environmental parameters.

The meter installation location must be chosen wisely. If you want to measure the background radiation, then you should take the counter to the street, and not just install it on the window. Concrete walls absorb radiation well, so a counter installed on a window will measure radiation only in the direction of the view from the window.
The effect of pressure changes on the count should also be taken into account for long-term measurements. With an increase in pressure, you can see that the count drops, and with a drop in pressure it increases.
This effect is compensated by the following formula: N = No * exp (k * (h-1000))
where No is the current count, h is the current pressure in millibars, k is the barometric coefficient for this counter.
The barometric coefficient does not affect the X-ray background, however, Geiger counters record for the most part secondary electrons and muons, which are subject to this effect. Therefore, the barometric coefficient must be calculated for different types of Geiger counters, since the composition of the radiation recorded by the counter is not exactly known. However, this is often difficult to do due to the low statistical stability of the account and the need to collect a large amount of data on “calm days” to accurately calculate the coefficient.
This necessitates the installation of a pressure sensor in parallel with the counter and calibration of the sensor.

In general, the main problem of Geiger counters is that they register charged particles (electrons and muons), and the efficiency of registration of radiation proper is not high. This can be partially solved by adding “covers” of metals of various thicknesses to the meter. However, then the question arises of calculating the thicknesses of these covers.

The main advantage of Geiger counters is their relative availability and a huge number of ready-made circuits and solutions for their connection.

Here is our assembly based on counters of 16 STS-6 counters arranged in two rows with an intermediate layer of aluminum:

2. Scintillators

Scintillator-based recorders are deprived of the above problems. General concepts about scintillators can again be read on Wikipedia. However, there are drawbacks.
First (and most importantly for us) scintillators are difficult to get. Secondly, a photomultiplier should be selected for the crystal. Thirdly, this PMT needs to be powered, and this is no longer 400 V as in geysers, but up to 1500 V, which is not so easy to obtain. Well, and finally, fourthly, this entire structure must be placed in a lightproof enclosure. But we do not need to think about barometric effects, since the high rate of gamma-ray counting clogs the electron count, which is confirmed experimentally. In addition, the scintillator operates in proportional mode, which means that, unlike the Geiger counter, it allows you to register not only the fact of the arrival of a particle, but also to measure its energy. This gives us the opportunity to build the energy spectrum of the radiation and if something radioactive (pah-pah-pah) falls on the sensor, then try to determine what it is from the spectrum.

However, we were lucky and we have a whole bunch of different scintillation detectors. Therefore, they were attached to the difficult task of dosimetry. Here are a couple of them (without additional electronics):



Data from continuous measurements over two years revealed some interesting patterns. For example, there is a one-year variation in background associated, as expected, with the release of Radon radioactive gas in the summer and the inability to release it from frozen ground in winter. One of the observation points, located near the coal mine, registers a gradual increase in background over time and sharp dips after precipitation. Coal dust is probably deposited on the sensor and washed off periodically by rain. And also an extremely interesting relationship was found between the detected gamma radiation and precipitation. Almost always, during precipitation, an increase in soft gamma radiation is recorded (which is just poorly recorded by Geiger counters). However, this is already a topic for scientific work ...

I did not cite electronic circuits, since most of them are quite trivial and can be found on the Internet. However, if necessary, I can supplement this article and give answers if the community finds the topic interesting.