# How to convert count rate into becquerels

1. Jan 28, 2008

### _Mayday_

I understand that the activity of a source is the number if disintegrations that take place per second in the source. But I do not know how to find the activity from the count rate and time.

My question is how do you convert count rate into becquerels? Is there a formula that I can use? Or I guess you could say CPM to DPM.

Thanks.

Last edited: Jan 28, 2008
2. Jan 28, 2008

### _Mayday_

I've just had a look around on the internet, and there doesn't seem to be any information on it that I have come across, so I don't know if it is very complicated or is just rarely done.

What I want to do is relate my count rate into the formula $$T^{1/2} = \frac{0.69}{\lambda}$$

3. Jan 28, 2008

### Staff: Mentor

Well adjusting for measurement efficiency (i.e. geometry, etc.) the count rate gives the activity.

becquerel (Bq) = 1 nuclear transformation per second (s-1).

An older unit of activity is the Curie, 3.7 x 1010 nuclear transformations per second, 3.7 x 1010 becquerels, or 2.22x1012 nuclear transformations per minute. The term nuclear transformations is often replaced by the term disintegrations.

The important thing to remember is that if the sample is not surrounded by the detector, then one is not detecting all the disintegrations, but only the fraction that interact with the detector. Even if the detector interacted with all the radiation, there is still the matter of the dead time, during which a one event is not detected because the detection system has not recovered from a previous event, i.e. two separate radiation events are counted as one.

This might be a good reference - Health Physics Society

Last edited by a moderator: Jan 28, 2008
4. Jan 28, 2008

### _Mayday_

Thank you, is what you are talking about at the end the efficiency of the experiment? As in obviously I am not able to measure all the radiation given off?

How would I go about calculating the decay constant? I would like to use the equation $$T^{1/2}=\frac{0.69}{\lambda}$$ to calculate the half life, I know that the logarithm on top is lm(2) but I do not know how to calculate the decay constant? I have measurements for Counts recorded on a GM tube, and the time intervals if that helps.

Last edited: Jan 28, 2008
5. Jan 28, 2008

### _Mayday_

I don't know if anyone understands what I am trying to say? Is the decay constant actually a constant and specific to a particular element? How can I find it then, I have looked everywhere.

6. Jan 28, 2008

### Staff: Mentor

If you want to measure the decay constant experimentally, you need to measure the counting rate as a function of time. For example, you might measure the number of counts in one minute, then wait nine minutes, then measure the number of counts in one minute, then wait nine more minutes, etc. The number of counts decreases with each measurement.

Make a graph of the number of counts versus time (in this example, t = 0, 10, 20, etc. minutes). Draw a smooth curve that more or less goes through the data points. From that curve you can estimate the time it takes for the number of counts to decrease by 1/2. This is the half-life, and you can calculate the decay constant from it using the formula you gave. There are more sophisticated ways of getting the half-life from the data, but this might be enough for a starting point.

In this setup you don't have to worry about the geometrical efficiency of the detector, because it will be the same for each data point, provided you don't move the source and the detector around.

7. Jan 7, 2009

### iman 11

Re: How to convert count rate into becquerels and calculated internal dose

IF WE give patinet 3mci of TC99M(DTPA) and put him in camma camera and take the count of kidney in different time

My question is how do i convert count rate into becquerels? Is there a formula that I can use? Or I guess you could say CPM to DPM.
then after we convert to activety we draw cruve of activity function time how can i calculated by it to now what the residence time in kidney to calculate internal dose?
are there the same if we want calculate the internal dose of kidney

Thanks

8. Jan 7, 2009

### QuantumPion

Don't forget self-shielding!

9. Jan 7, 2009

### QuantumPion

Re: How to convert count rate into becquerels and calculated internal dose

A Becquerel/Curie is just a measure of the number of disintegrations per second of the sample and is only dependent on the isotope and how much of it there is. A count rate is how many of those disintegrations your instrument manages to detect, and is dependent on the detector efficiency and geometry. To convert count rate to becquerels you would need to get information about the detector you are using (type/model, often it has its detector characteristics printed on it), as well as biological shielding effects from a HP manual.

To determine the dose to an organ is more complicated. I'm not a health physicist so I don't know all the details. You would need information such as energy spectra, source distribution, biological density and composition, etc. There are probably tables or manuals that tell you how to do this.