[ RadSafe ] Tc-99 disc problem. My attempt at solving the problem.

Geo>K0FF GEOelectronics at netscape.com
Sun Dec 16 21:16:41 CST 2007

Thanks for your participation.
Tc-99 is one of the purest monoenergetic beta emitters found in the test laboratory environment,

with a  0.999990 probability of a 0.293600 MeV Max (0.084600 MeV average) Beta particle per disintegration.

Our question was "What is the surface emission rate a 27 pCi Tc-99 test disc, electroplated on a stainless steel substrate in betas per minute?" The information being requested is the 2 pi beta emission rate from the source delineated.
Another way of stating it is: the total number of beta particles per minute coming from the front working surface of the disc on the calibration date. 
Usually an old source would also have the calibration date and the present measurement date specified, but for Tc-99 the half-life is 213 thousand years ( Hacker, Radiation Decay program), so it is a moot point for our exercise.

Forget about self absorption, counting error, confidence level, and
systematic error. We are not concerned with a detector, we want to know how many betas are coming off the test disc.
We are not dealing with a detector yet, only the disc. The detector efficiency is another measurement, and it is the main one we own the Tc-99 disc for, in our case. 

This was not a trick question. To determine the beta efficiency of a probe,
one needs to know how many particles are leaving the test disc. An everyday routine measurement for some of us. 

NIST traceable disc beta sources are specified in surface emission using "surface CPM". As the name implies, it is the CPM from the surface of the test disc.By having such a specification, efficiency factors, penetration coefficients and other basic measurements may be easily performed.
Disintegrations per minute (DPM) of any source that has the activity level given (as: 27 pCi) is easily calculated by multiplying 2.22 times the activity number and applying the correct magnitude multiplier. 

27 pCi= 59.94 DPM, 4 pi geometry. That just means there are that many total disintegrations, going in all directions. 

To ascertain the SURFACE CPM, the first step is to divide the 4 pi CPM by (2) to derive the 2 pi CPM, since only that portion is being presented to the front surface of the disc.

If we were dealing with Gamma Rays, that would pretty much be the end of the calculations.

A peculiarity of Beta Particles ( both the positive kind and the negative kind) is that they have a tendency to bounce off objects. In our lab, tests have shown clearly that the bounce factor has a lot to do with the A number of the material which is causing the bounce. In the subject disc, so we made it perfectly clear that the substrate is Stainless Steel. This material has a bounce factor of 25%. Another material would be higher or lower depending upon the atomic density of the material.

Correct technical term for bounce factor is "backscatter factor". 25% of the beta particles that originally were heading towards the substrate bounce off, adding to the forward flux.

So we know that the forward scatter or 2 pi CPM is 59.94/2= 30 (rounded). The backwards facing radiation is the same. One fourth of the backwards facing radiation will bounce of the substrate, or BACKSCATTER from the substrate, winding up going out the front, adding to that portion that is going out the front anyway.

So .25 X 30= 7.5. Then  7.5 plus 30 = total Surface emission rate of  37.5 (rounded) particles per minute.

Lastly we must determine any self absorption factors, both for the forward scatter and the backscatter. The subject disc is know to be electroplated, implying a very thin layer. We can then imply that there is no significant self absorption. 

Now that we know the surface emission rate in betas per minute, 37.5, it would be easy to use the test disc to determine the Beta Efficiency Factor of a pancake or other probe, for the stated energy levels, a common task in the Rad Lab. Typically the probe would also be tested with P-32, C-14 and Sr-90/Y-90 energies as well. 

Using the rule of thumb for Beta Distance in Air as 12 feet per MeV, and an average Beta energy of 0.084 MeV, the average distance the Tc-99 Beta will travel in air is about 1 foot. 

Betas can be deflected by a horseshoe magnet, 90 degrees is easy even with a toy magnet. As such, all measurements protocol must keep in mind any stray magnetic influences. 

Parameters of source shape, disc substrate material, being electroplated, and the backscatter factor are all very relevant to the successful solution of the problem.

If a listmember finds this kind of information useful, drop me an eMail. Likewise if it is so well known as to be silly, let me know that too.

George Dowell
New London Nucleonics Lab

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