Theory of Radiation Monitoring

This topic is in fact the answer to the topic Continuous monitoring of radiation background in Moscow . I hope it will help those who wish to organize their own monitoring.

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

1. Geiger counters

You can read about the device of the Geiger counter in 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 the background bill.
The wall thickness depends on the brand of the meter and determines its effectiveness in registering radiation. In this regard, the SBM-20 and SBM-21 counters, which are easily extracted from Soviet dosimeters, are the most suitable for our purposes. Also often come across counters such as STS-5. They have too thin walls, but you can wrap them with a ~ 0.5mm foil and you get a decent detector. Otherwise, it will count secondary electrons from the environment.
The background (natural) count of the detector should be fairly low. Usually 25-50 pulses per minute. A higher score leads to poor statistics and difficulty in registering small variations. For example, in the CTC-6, the background bill is already 110 imp / min. However, this can be dealt with by including several parallel meters in parallel with the output of either a total account or averaging data from several registration channels.
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1.2. Environmental parameters.

The installation location of the counter must be chosen wisely. If you want to measure the radiation background, then you should take the meter 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 bill should also be taken into account in long-term measurements. With an increase in pressure, it can be noted that the score 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 register for the most part secondary electrons and muons, which are subject to this influence. Therefore, the barometric coefficient must be calculated for different types of Geiger counters, because the composition of the radiation recorded by the counter is not precisely known. However, it is often difficult to do this due to the low statistical stability of the account and the need to collect a large amount of data on “quiet days” to accurately calculate the coefficient.
This necessitates installation in parallel with the pressure sensor counter and the calibration of this sensor.

In general, the main problem of Geiger counters is that they register charged particles (electrons and muons), and their radiation registration efficiency is not high. In part, this can be solved by adding “covers” of metals of different thickness to the counter. However, then the question arises about the calculation of the thickness of these covers.

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

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

2. Scintillators

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

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



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

I did not give 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.