The Geiger-Mueller Counting System

The operation of the Geiger-Mueller (GM) counter is based on gas-filled radiation detector which detects the energy released by the charged particles in the gas, which occurs when the electron-ion pairs are created. This means that the radiation directed into the gas ionizes the atoms of the gas in the GM tube (Korff, 2013) . The detector has an active portion that is made up of a metal cylinder that is filled with the counting gas. These counts are then recorded using either of the two given methods: the cinder has a thin window at its end where the radiation of beta and alpha particles enter the gas, which is the counting region. In the process of this collision, the ion-pairs are created, swept away in the GM tube using electric field, and are then counted (Korff, 2013) . The application of GM tube in the x-rays or gamma rays is such that there is an interaction between the metal cylinder and the photons, leading to the creation of secondary charged particles, or the electrons in this case, and results in the creation of ion pairs as in other cases. 

The aim of this experiment was to calculate the absorbed dose, which results from the ionization of radiation, and is used in everyday life, especially in the health sector, but may pose health risks such as cancer. To achieve this calculation, the type of ration emitted by the source was considered as gamma, alpha, beta or neutron. Again, the energy of the photons, the number of photons emitted per unit time by the source and the target geometry in relation to the source of the radiation used in ionization was also considered. 

Procedure 

The experimental setup was as shown in figure 1 below 

Figure 1: Experimental setup 

At high voltage point, the HV power was turned on by clicking on the power symbol, and the voltage KV increased to 0.90 using the arrow. At the Source point, the source was right-clicked on, the properties, in order to change the activity at the bottom of the page to 37,000 Bq (1uCi). This was then updated by clicking on “update” icon. Using the bottom screen, the detector Y coordinate and the source was positioned around 10cm apart. At the amplifier point, the fine gain was set at minimum, the course gain set at 110, while the polarity was set at negative, and finally the output set as unipolar. Lastly, the setup of the counter point was set by double clicking the “000” icon and entering time in seconds. Part of the simulation setup was as shown in figure 2. 

Using the “triangular run” icon, the experiment was set in motion and the measurements obtained recorded. The counter registered all the counts and stopped at specific times as set in the experiment. The radiations emitted from the GM counter were then counted over a set period of time from a radioactive source. The source was counted for 60 seconds and the total counts recorded. This was then repeated for 300 seconds, 900 seconds, and 120 seconds, and the total counts registered were recorded. 

Figure 2: Part of the simulation setup 

Results 

The data collected from the experiment, as well as the other calculations involved were recorded as shown in table 1 

Table 1: Data collection and calculation 

The valued obtained from beta energy were used to plot a graph showing its amplitudes measured at different sources as shown in figure 3 

Figure 3: Measured Beta amplitudes at various sources 

Discussion 

From table 1, the total number of counts is more than the three different sources used, since it is the summation of the three sources. Again, the counting rate averaged from the three sources is 43.84 CPS, which is slightly more than the first two sources, and less than the third source. The source counting rate per minute is measured by dividing the number of counts per source by the counting time, converted from second to minutes. It was noted that during the two-minute count when no radiation is present, the GM machine still registered some counts, albeit at low values. This is due to the presence of the radiative materials in the earth nearby, which could be detected by the GM machine, leading to the count. Some of the sources of these radiations in everyday life include the air we breathe, the food we eat, building materials used in the homes and the cigarettes that people smoke among others. However, the measured value from such counts is low, since the radiations are not as active as the sources used in the GM tube. 

The background radiations from the mentioned sources in everyday life affect the accuracy of the counts obtained from the sources. In order to counter the error that results from this inaccuracy, the value of the background radiation is first recorded, and then subtracted from each value obtained from the sources, so as to obtain the accurate value of the radiations from the sources (Kolb, 2014) . Figure 3 shows the amplitudes obtained from the beta radiation sources. The decay of energy seems to be far much fluctuated from the sourced as seen by the pulse heights measured. It is thus possible to conclude from this point that the amount of ionization required to initiate radiation event does not depend on the pulse height from the GM counter (Henriksson, 2018) . Again, the distribution of the pulse heights can be affected by the randomness of the gas molecules quenched in the process, albeit only to a slight extent. The positive heavy ions are clustered inside the tube which is used to achieve ultimate termination of the avalanching effect. This can result in the delay when allowed to do so, thereby developing pulse heights which are slightly larger. 

The aim of this experiment was to calculate the absorbed dose, which results from the ionization of radiation. This has been achieved by determining the radiation counts per minute and per second, as well as the background radiations. The results obtained were tabled and the required calculations done. Overall, the objective was achieved with slight errors resulting from the background radiations. 

References 

Henriksson, M. (2018).Photon-counting panoramic three-dimensional imaging using a Geiger-mode avalanche photodiode array.  Optical Engineering ,  57 (09), 1. https://doi.org/10.1117/1.oe.57.9.093104 

Kolb, K. (2014). Signal-to-noise ratio of Geiger-mode avalanche photodiode single-photon counting detectors.  Optical Engineering ,  53 (8), 081904. https://doi.org/10.1117/1.oe.53.8.081904 

Korff, S. (2013). How the Geiger Counter started to crackle: Electrical counting methods in early radioactivity research.  Annalen Der Physik ,  525 (6), A88–A92. https://doi.org/10.1002/andp.201300726