We have previously written about how to calculate the coefficients for the SBM20 tube and the J305, tube that come with our Geiger counter module GGreg20_V3 for DIY / IoT projects.
However, in those articles, we focused heavily on the calculation formulas and almost overlooked a very important detail: when calculating the conversion factor for the Geiger tube pulse count, we need to be aware of what exactly we want to get as the output.

If we need to obtain the radiation power of a radioactive source registered by the counter, this is one task.
It is a completely different task when we need to obtain the equivalent value of the radiation dose absorbed by the human body over a certain period of time.

We describe in detail how to calculate the coefficients in our previous publications, so we will not waste the reader’s time now.
Instead, we will try to show the differences between the calculated coefficients and how best to use them for a DIY project.

The data is provided for the J305 tube. Any other Geiger tube, such as the SBM20 or LN712, can also be used in its place, since they all have a similar principle of operation except for certain nuances that we can neglect for the purposes of this discussion.

In our previous publications, we went from start to finish: we have pulses -> we want to keep the value in μSv/h.
Today we will try to go the other way: from the goal of the DIY project through the sequential steps to achieve it.

As DIY Geiger counter users, we would most likely want to have access to the following information on our device’s display:

1. CPM: The number of registered pulses per minute received from the counter;
2. μSv/h: The radiation power of the source registered by the counter;
3. μSv: The absorbed dose by the human body over a certain period of time.

Take a close look at these values. The secret to calculating them correctly lies not only in the correct formulas and coefficients, but also in understanding the overall process:

• The source emits radiation;
• The counter registers it;
• The human body absorbs it.

The human body does not absorb everything that the counter registers. The counter readings and the absorbed dose are highly dependent on many factors. Therefore, to simplify the measurements, it is common to use ready-made data from the manufacturer and equivalent models for calculating the corresponding coefficients.

Pulse Count

We obtain the pulse count from the Geiger counter module. For convenience of calculations, as well as based on the pulse density during normal background radiation measurements, this parameter is best calculated in counts per minute (CPM).

For example, the J305 manufacturer specifies in the datasheet that the tube should emit 25 pulses per minute under normal background radiation. In other words, 25 CPM.

Why do we need the CPM value on the display if there are other, more understandable indicators? Indeed, it is possible to do without pulses per minute. However, it is a very convenient indicator when we want to understand if there are any malfunctions in the device. Usually, in our devices, we make a cumulative counter of the number of registered pulses for the entire time the device has been operating since the power was applied.

In this case, even if we do not have a source of accurate synchronized time, such as NTP or RTC, we can still calculate the average number of pulses per minute by dividing the sum of all pulses by the total time elapsed since power was applied. This indicator can indicate the quality of our data, even if hours have passed since the device was turned on.

We hope that since the principle of such checks is already known to you, you will develop your own algorithms for checking the data quality and the operation of the counter if necessary. There can be many implementations.

Radiation Source Power

When we want to estimate the radiation source power, we need to apply the conversion factor from CPM to μSv/h obtained by calculation based on the data specified in the tube’s datasheet from the manufacturer:

For the J305 tube, the manufacturer specifies a sensitivity of 44 cpm per 1 μR/h from a Co-60 source;

Let’s convert the data we need:

1. Converting to counts per minute at 1 mR/h:

CPS / mR/h → CPM / mR/h: 44 * 60 = 2640;

2. Converting to counts per minute at 1 μSv/h:

CPM / mR/h → CPM / μSv/h: 2640 / 10 = 264;

3. Value of one pulse per minute in μSv/h:

1 / ( CPM / μSv/hr ) = 1 / 264 = 0.00378;

Therefore, if we need to convert the pulses registered by the J305 tube during a minute to μSv/hour and obtain the radiation source power value:

1 CPM = 0.00378 [μSv/h];

or

When we read the documentation for a Geiger tube, this is the parameter that is usually discussed.

Equivalent Dose Absorbed by the Human Body

To obtain the value of the equivalent dose of radiation absorbed by the human body, we will apply the model of a human body phantom:

1. Lets convert counts per second to counts per minute at 1 mR/h:

44 * 60 = 2640 pulses/minute / mR/hour

2. Convert CPM at 1 mR/h to CPM at 1 R/h:

2640 * 1000 = 2640000

3. Let’s find the value of the exposure dose R/h at 1 CPM:

1 / 2640000 = 0.0000003787878788

4. Find the air-kerma (Ka, kinetic energy released per unit mass/in matter):

The equation is as follows:

Ka [Gy] = 0.00877 [Gy/R] x exposure [R]

where 0.00877 – radiation dose absorption coefficient by the human body on a phantom model under the influence of photon energies of 100 keV – 3 MeV

0.00877 * 0.0000003787878788 = 0.000000003321969697 Ka[Gy]

5. Let’s convert Ka[Gy] to Ka[μSv] (i.e., from Gray to μSv):

0.000000003321969697 * 1000000 = 0.003321969697 Ka[μSv]

Thus, the formula for the equivalent absorbed dose of radiation by the human body for the Geiger-Muller J305 tube with gamma sensitivity for Co-60 of 44 cps/mR/h is as follows:

Please note: To obtain the cumulative value of the equivalent dose of energy absorbed by the human body, we need to add the hourly values to the previous accumulated sum throughout the entire measurement period (from the moment of power supply or resetting the counter value). See the example below.

Why should we be more interested in the dose of radiation absorbed by the human body than in the radiation source power? There are at least two reasons for this, which follow from each other.

First, from the point of view of radiation safety, it is not the power of the source itself that is important, but the dose that we will absorb over a certain period of time if we are exposed to the radiation source.

Second, since the dose of absorbed radiation is so important, it is the dose for humans that is calculated and provided by government organizations for radiation protection and public health.

Usually, the permissible dose is given for a period of one year. It is such tabular data that allows us to objectively assess what is the normal level of background radiation – the moment when the radiation source power and the equivalent dose absorbed by the human body can be converted to common units and compared.

If the normal background level, in terms of its instantaneous power converted to a year, turns out to be higher than the permissible dose per year, then this may not be background, but a certain radioactive source. And then something needs to be done immediately with the radiation source that creates such a “background” so as not to exceed the permissible absorbed dose of radiation by our body.

Practice and Examples

Now we come to the most important thing: when creating sensor entities for GGreg20_V3 with a J305 tube, for example in ESPHome, we need to use the appropriate coefficients for different physical values.

1. Number of pulses per minute is always just the number of pulses – a dimensionless value. But if necessary, a conversion or averaging coefficient can also be applied (not considered in this article);
2. For radioactive source power: CPM * 0.00378 [μSv/h];
3. For the equivalent dose absorbed by the human body: CPM x 0.00332 [μSv/h], finding the sum of the minute-by-minute values [μSv] for the entire measurement time.

Example for ESP32 + GGreg20_V3 + J305 у ESPHome

# 1. Pulse Count Sensor from a Counter with a 60-Second Measurement Cycle

 sensor:
- platform: pulse_counter
 pin:
 number: 23
 inverted: True
 mode: 
 input: True 
 pullup: False
 pulldown: False
 unit_of_measurement: 'CPM'
 name: 'Ionizing Radiation CPM'
 count_mode: 
 rising_edge: DISABLE
 falling_edge: INCREMENT # GGreg20_V3 uses Active-Low logic
 use_pcnt: False
 internal_filter: 180us # for J305
 update_interval: 60s
 accuracy_decimals: 0
 id: my_cpm_meter

# 2. Radiation Source Power Sensor with a 60-Second Measurement Cycle

- platform: copy
 source_id: my_cpm_meter
 unit_of_measurement: 'uSv/Hour'
 name: 'Ionizing Radiation Power'
 accuracy_decimals: 3
 id: my_power_meter
 filters:
 - multiply: 0.00378 # for J305

# 3. Sensor for Equivalent Dose Absorbed by the Human Body per Hour with a 60-Second Measurement Cycle

- platform: copy
 source_id: my_cpm_meter
 unit_of_measurement: 'uSv/Hour'
 name: 'Ionizing Radiation Equivalent Absorbed Energy'
 accuracy_decimals: 3
 id: my_dose_meter
 filters:
 - multiply: 0.00332 # for J305

# 4. Cumulative Equivalent Dose Absorbed by the Human Body from Radiation Since the Start of Measurement (i.e., from the Moment of Power Supply)

- platform: integration
 name: "Total Ionizing Radiation Equivalent Absorbed Energy Dose"
 unit_of_measurement: "uSv"
 sensor: my_dose_meter # link entity id to the pulse_counter values above
 icon: "mdi:radioactive"
 accuracy_decimals: 5
 time_unit: min # integrate values every next minute
 filters:
 # cumulative absorbed dose. Converting it from uSv/hour into uSv/minute: [uSv/h / 60] OR [uSv/h * 0.0166666667]. 
 - multiply: 0.0166666667
 # but if my_dose_meter in CPM, then [0.00332 / 60 minutes] = 0.000055; so CPM * 0.000055 = dose every next minute, uSv.
 #- multiply: 0.000055 # for J305

Conclusions

In this article, we made an important discovery that the coefficients for different tasks for the same Geiger tube should be different. This approach differs from the examples that are given on the Internet.

Although we have already written in passing about different coefficients (see articles on calculating conversion coefficients), in our numerous examples on GitHub , we previously used only one coefficient, because that’s what everyone does.

Now we believe that it is better to calculate a separate coefficient for each sensor entity depending on its type and purpose. Perhaps we will even update the YAML files in our examples on GitHub.

References to other publications and examples

UA: Geiger tube J305: How to calculate the conversion factor of CPM to μSv/h Technical note
EN: Geiger tube J305: How to calculate the conversion factor of CPM to μSv/h. Technical note

UA: Технічна нотатка: Як розрахувати коефіцієнт перетворення для трубки Гейгера СБМ20
EN: Technical note: How to calculate the conversion factor for Geiger tube SBM20

UA: Трубки Гейгера-Мюллера: порівняння SBM20, J305 та LND712
EN: Geiger-Muller tubes: Comparison of SBM20, J305 and LND712

EN: DIY Geiger counter GGreg20_V3 on GitHub

Easy Links

Unique Vendor ID: https://go.iot-devices.com.ua/ggreg20_v3
User Friendly ID: https://go.iot-devices.com.ua/geiger-counter

Where and how to order

Website Online Shop
Etsy Store
Tindie Marketplace

There are a lot of publications on the Internet on how to convert CPM (Counts per Minute) obtained from a Geiger tube to radiation levels. However, despite the wide coverage of this topic on various forums and a wide range of examples with code for various programming languages, we had to understand the topic deeper than we wanted to in order to be able to properly calculate the conversion factor for Geiger tube SBM20 from CPM to absorbed dose of radiation μSv/h and configure the conversion in our own products and make an example calculation for everyone

IoT-devices, LLC is producing its Geiger counter module GGreg20_V3 with a pulse output and two types of tube to choose from. Currently, customers can choose either SBM20 or J305 when ordering a device from our production.

We plan to create separate documents for each of the popular tubes, and we are starting with a technical note on SBM20.

Therefore, in this document, we will look at the procedure for calculating the CPM to microsieverts per hour conversion factors for SBM20. For other common tubes, at least J305 and LND712, it will be covered later.

While collecting materials for this article, we came to the conclusion that this is not an easy task at all, because information is spreading on the Internet, which needs to be verified, and sometimes we even had to investigate where certain coefficients came from.

Let’s start with the useful information that manufacturers provide in their datasheets for their tubes.

Solution

Let’s make calculations for SBM20 based on the manufacturer’s data:

Count rate at 4 μR/s from a Cs-137 source of pulses per second: 240 – 280;

1. Let’s take the average of these two values:

This is the average count rate at 4 μR/s from a Cs-137 source of pulses per second.

(240 + 280)/2 = 260 CPS / μR/s

2. Convert to pulses per second at 1 μR/s: μР/с:

260 / 4 = 65 CPS / μR/s

3. Convert to pulses per second at 1 mR/s:

65 * 1000 = 65000 CPS / mR/s

4. Convert to pulses per second at 1 mR/h:

round( 65000 / 3600) = 18 CPS / mR/h

5. Convert to pulses per minute at 1 mR/h:

18 * 60 = 1080 CPM / mR/h

6. Convert to pulses per minute at 1 μSv/h:

1080 / 10 = 108 CPM / μSv/h

or

108 CPM = 1 μSv/h

or

1 CPM = 1/108μSv/h

7. Calculate the value of one pulse per minute:

1 / 108 = 0.00926

Thus, if we need to convert the pulses recorded by the SBM20 tube during a minute into μSv/hour:

This is the value of the equivalent radiation dose recorded by the sensor – the SBM20 tube.

Please note that we took the average value of CPS / μR/s = 260 and obtained the value of CPS / mR/h = 18 by simple mathematical transformations.

We can also perform the above calculations not only for the average, but also for the minimum and maximum values specified in the data sheet for the tube: 240 CPS and 280 CPS at 4 μR/s.

In this case, we will get two additional values that we can also work with if necessary.

For convenience, let’s write them in CPS at 1 mR/h:

min value: 17 CPS / mR/h
max value: 19 CPS / mR/h

Let us summarize the results of our work at this stage of the calculation:

Everything would have been fine, and we could have stopped calculations there. But the coefficient we calculated, 0.00926

0.00926

only allows us to obtain the exposure value recorded by the Geiger counter. We are primarily interested in the equivalent dose of radiation absorbed by the human body.

Therefore, let’s move on to the next part of the calculations.

Although we are not a research institute, to solve our simple task we will have to dive into complex matters for a while.

In order to estimate the equivalent dose of energy absorbed by the human body, science uses the so-called human body phantom model, which calculates certain conversion factors for converting one value to another.

If you are interested in reading the theory on this subject, we can advise you to read this publication:

https://web.archive.org/web/20230402162906/https://www.automess.de/en/service/radiation-quantities-and-units

And we proceed to derive the conversion factor of the tube CPM into the equivalent dose of absorbed radiation in microsieverts per hour, taking into account the phantom model of the human body.

Let’s start the calculation with the already found coefficient of 18 CPS per 1 mR/h. Why is this so? Next, you will see that most tube manufacturers (J305 and LND712 included) provide parameters in this format in the datasheet for their products.

For example,

• for the J305 is specified in the datasheet: sensitivity of γ (60Co) cps/ mR/h 44
• for LND712: GAMMA SENSITIVITY CO60 (CPS/mR/HR): 18

where CPS – Counts per Second; mR – milli roentgen; h = hr – hours.

In the case of the SBM20 tube, the manufacturer specifies data for the Cs-137 source.

And we have already made the necessary conversions in the previous step:

Sensitivity of SBM20 to gamma rays: 18 CPS / mR/h 1.

1. Convert CPS to CPM at 1 mR/h (we already have this value, but we are going to calculate it again for the reader’s convenience):

18 * 60 = 1080 CPM / mR/h

2. Convert CPM at 1 mR/h to CPM at 1 R/h:

1080 * 1000 = 1080000

3. Find the value of the exposure dose R/h per 1 CPM:

1 / 1080000 = 0.0000009259259259

4. Find the air kerma (Ka, kinetic energy released per unit mass / in matter):

The equation is as follows:

Ka [Gy] = 0.00877 [Gy/R] x exposure [R]

where 0.00877 is the coefficient of radiation dose absorption by the human body on the phantom model under the influence of photon energies of 100 keV – 3 MeV

Note. For more details, see the link: https://web.archive.org/web/20230402162906/https://www.automess.de/en/service/radiation-quantities-and-units

0.00877 * 0.0000009259259259 = 0.00000000812037037 Ka[Gy]

5. Convert Ka[Gy] to Ka[uSv] (i.e., switch from Gray to µSv):

0.00000000812037037 * 1000000 = 0.00812037037 Ka[uSv]

6. Perform the check and find the inverse value:

0.00812037037 ^(-1) = 123.1470924

Thus, the formula for the equivalent absorbed dose of radiation by the human body for the Geiger-Muller tube SBM20 with gamma sensitivity for Cs-137 of 18 CPS / mR/h is as follows:

where

0.00812 μSv/h is the value of one count, CPM;

CPM – number of counts per minute.

We can also calculate the coefficients for the minimum and maximum CPS / mR/h values provided by the datasheet:

At the same time, the Internet uses a coefficient of 175.43 (0.0057), which, as can be seen, does not fit into the calculations above. However, this coefficient is similar in value to the data for SBM20 for Ra-226 taken from the Internet:

In order for the CPM to uSv/h conversion factor to be 0.00570, the initial value of CPS/mR/h in the tube datasheet should be 25.643. We determined this by the simple value adjustment (25.643 * 60 / 8.77 = 175.4367; 1 / 175.4367 = 0.00570).

Another popular coefficient for the SBM20 tube is also published on the Internet: 150.5131 (0.00664), allegedly calibrated to Co-60. We do not know where this information came from.

Additional calculations

What else can we do to try to find the 0.0057 coefficient that is used everywhere for SBM20?

Switching from coefficients for Cs-137 to Co-60

We can try to move from the values at Cs-137 (which is indicated in the tube documentation) to Co60, which is now the most commonly used standard for calibrating Geiger-Muller tubes in the world.

To do this, it is necessary to calculate the conversion factor for Cs-137 to Co-60 values.

Let’s take the average value for the two Co-60 energy lines:

Co-60: 1.1732 MeV; 1.3325 MeV; Average value: 1.25285 MeV

And the energy value of Cs-137:

0.6617 MeV

Let’s find the energy ratio of Co-60 to Cs-137:

1.25285 / 0.6617 = 1.893380686

Next, we calculate the corresponding value of CPS at 1 mR/h:

18 * 1.893380686 = 34.08085235

Assuming that this value is indicated in the data sheet for the tube, we will calculate the new coefficient of the absorbed equivalent dose of μSv/h per 1 CPM for Co-60:

34 CPS / mR/h

1. Convert CPS to CPM at 1 mR/h:

34 * 60 = 2040 CPM / mR/h

2. Convert CPM at 1 mR/h to CPM at 1 R/h:

2040 * 1000 = 2040000

3. Find the value of the exposure dose R/h per 1 CPM:

1 / 2040000 = 0.0000004901960784

4. Find the air kerma (Ka, kinetic energy released per unit mass / in matter):

The equation is as follows (as in the previous calculation):

Ka [Gy] = 0.00877 [Gy/R] x exposure [R]

So:

0.00877 * 0.0000004901960784 = 0.000000004299019608 Ka[Gy]

5. Convert Ka[Gy] to Ka[uSv] (i.e., switch from Gray to µSv):

0.000000004299019608 * 1000000 = 0.004299019608 Ka[uSv]

uSv/h = CPM x 0.00429 for Cs-137->Co-60

Switching from coefficients for Cs-137 to Ra-226

We calculate the conversion factor from Cs-137 to Ra-226 in a similar way:

Because Ra-226 has several energy lines of gamma radiation:

186.2 keV;
240.3 keV;
295.2 keV;
352.0 keV;
609.3 keV;
657.0 keV;
768.4 keV;
934.8 keV.

we will select all of them that can be detected by the SBM20 tube. According to the documentation, the SBM20 is sensitive to gamma quanta from 0.05 MeV to 3 MeV.

Therefore, all of these lines fall within the detection range. In order to move from Cs-137, we need to either work with the average of these lines or select the line most characteristic of Ra-226.

If we need to rely on a specific line to identify a radionuclide, we need to take into account the intensity and energy of this line. Usually, the so-called “characteristic peak” is used to identify a radionuclide, which is the most intense and is located at the most characteristic energy for this radionuclide.

In the case of the gamma spectrum of Ra-226, which consists of several lines, the characteristic peak can be determined by the highest intensity and energy of the line. According to the table of line intensities of the Ra-226 gamma spectrum, the line with an energy of 609.3 keV is the most intense line, so this line can be used to identify Ra-226.

The average value of the energies of all lines of the gamma spectrum of Ra-226 can also be determined, but it is not useful for identifying the radionuclide. However, the average value is useful for determining the average gamma energy of a given radionuclide.

The table of line intensities and energies can be used to calculate the average gamma energy of Ra-226.

Let’s calculate the sum of the energies of all lines of the Ra-226 gamma spectrum:

0.186 MeV + 0.244 MeV + 0.295 MeV + 0.351 MeV + 0.609 MeV + 1.061 MeV + 1.158 MeV + 1.332 MeV = 5.196 MeV

Divide the sum of energies by the number of lines to get the average energy:

5.196 MeV / 8 = 0.6495 MeV

Thus, the average gamma energy of the Ra-226 radionuclide is approximately 0.65 MeV. Note that this value is only an average energy and may differ from the individual values of the gamma spectrum lines.

Now we have three alternative values that we can use to find the conversion ratio from Cs-137 to Ra-226 calibration. The third option is the weighted energy value provided on the hps.org website.

1. Characteristic peak: 609.3 keV [peak];

2. Average energy of gamma radiation: 0.6495 MeV [avg];

3. Weighted value of gamma radiation energy: 0.74 MeV [wght].

See. https://web.archive.org/web/20230404222401/https://hps.org/publicinformation/ate/q4817.html

Let’s calculate the corresponding energy ratios:

Cs-137 energy value: 0.6617 MeV.

Find the energy ratio of Ra-226 and Cs-137:

peak: 0.6617 / 0.6093 = 1.086000328245528
avg: 0.6617 / 0.6495 = 1.018783679753657
wght: 0.74 / 0.6617 = 1.118331570197975

Next, we will calculate the corresponding value of CPS at 1 mR/h, i.e., we will move from the Cs-137 calibration, as indicated in the datasheet, to the calculated calibration value relative to Ra-226:

peak: 18 * 1.086000328245528 = 19.5480059084195
avg: 18 * 1.018783679753657 = 18.33810623556582
wght: 18 * 1.118331570197975 = 20.12996826356355

As we can see, the attempt to switch from Cs-137 to Ra-226 did not give us anything either, because we could not get the target CPS value of 25.643/mR/h, at which it is possible to obtain a coefficient of 0.00570.

The coefficient of 0.00429, which we obtained earlier for the case of Cs-137->Co-60, is also not similar to the common one for SBM20 175.43 (0.0057). Therefore, we can only guess where the Internet contributors got it from and use the one we obtained above by calculations based on the information in the datasheet for the tube.

Note . If you thought that we did not check the datasheets of the modern production SBM20 and only rely on outdated Soviet data, this is not the case. We have checked all possible sources available on the Internet. SBM20 tubes produced in 2021 and later have different calibration options in their documentation:
– 78 imp/μR without specifying a source (1/150.5131129, or 0.006643);
– 105 imp / µR at Ra-226 (1 / 198.4036488, or 0.005040);
– 67.5 imp/μR at Cs-137 (1 / 129.9885975, or 0.007692);
None of the documents we studied contains source data that could lead us to the coefficient of 0.0057 μSv/h at 1 imp/min.

As a reminder, our recommended formula for the equivalent radiation dose absorbed by the human body for the Soviet Geiger-Muller SBM20 tube with a gamma sensitivity of 18 cps/mR/h for Cs-137 is as follows:

where

0.00812 μSv/h is the value of one count, CPM;
CPM – number of counts per minute.

Conclusions

In this article, we have provided a detailed step-by-step calculation of the conversion factor for the data transmitted by the Geiger counter with the SBM20 tube and figured out what factor we need to apply in order to not only get the level of exposure dose recorded by the sensor, but also the dose absorbed by the human body.

Unfortunately, we could not figure out where the magic coefficient of 0.0057 μSv/h per 1 CPM, which everyone uses, came from. Neither mathematical transformations, nor the transition from Cs-137 to Co-60, nor the transition from Cs-137 to Ra-226, nor the adjustment of the coefficient (in particular, 8.77, 0.94 to 0.98) of the absorption dose for the phantom model of the human body gave the desired results. It seems that this is a special case that someone, at some point, simply accepted their calculations and did not leave instructions for us.

Please write to us if you have your own version of where 0.0057 came from or a good calculation on this topic. We will be grateful and will make appropriate additions to this post.

At the same time, we were able to find a mathematically sound calculation of another magic coefficient

8.77,

which is used to obtain the value of the radiation dose absorbed by the human body. Now you also know where it comes from.

Currently, for soviet tubes, we recommend that instead of the coefficient of 0.0057, use the coefficient that we calculated for the equivalent dose of radiation absorbed by the human body:

where

0.00812 μSv/h is the value of one count (CPM) for the SBM20 tube calibrated against the Cs-137 source;
CPM – number of counts per minute.

If your SBM20 tube has a different sensitivity factor in the documentation, or if your tube is calibrated by the manufacturer against a different radioactive source, we recommend that you use the data for your tube.

In the next publications for J305 and LND712 tubes, there will be a little less text, because almost all coefficients published on the Internet correspond to our calculations.

And we have not yet covered the topic of internal background (false-positive) pulses for Geiger tubes. The work is not over yet!

Stay tuned!

Thank you for your attention!

Team IoT-devices, LLC