SBT-11A test and Calculating Dead Time

If all works as it should, the attached Excel 2016 formula will calculate a G-M tube dead time. For the SBT-11A that I have been using, it is

rather discouraging. 385.
You need two sources giving more than 20,000 counts per minute. I use two smoke detector americium buttons. Unfortunately, two of these saturate the SBT-11A, so I added another layer of stainless steel mesh to diminish the counts. Probably should have used another piece of mesh.

First, 2 buttons are positioned and taped down on the mesh and readings taken. Then, one button was left in the same location and readings taken. Last, the second button in it's original position. Each time take at least 1 minute of readings. I used 4 minutes for the single button readings and only two for the pair.

Bryan Boardman of Aware Electronics gave me this Excel solution a few years ago.

Here goes again! This editor is the pits for navigating and making attachments.
This is a Dead Time Calculator, Excel 2016, in DTC.RAR.

Okay folks, I ran the procedure once again, but this time I used tape and fixed the americium buttons along with the SBT11A. Had the tube mounted in a metal box with Aware Electronics HV/discriminator PCB. Running AW-RADW software. First, I did both buttons for over 8 minutes, then lifted the tape and flipped off one button for several more minutes (thanks to a phone call). Finally, I resecured the button and flipped off the other for 10 minutes.

I took an 8 minute average for each reading. Button A, button B and buttons A+B. Just copied the text numbers into Excell and used a formula in A2, B2 and C2, =average(d1:d2) etc.

This time, I put the =abs() function into Bryan's formula.

Averages were as follows:

Buttons A+B 241Am buttons = 68,660 CPM
Button A = 41,983 CPM
Button B = 28,275 CPM

Deadtime in Microseconds uses the formula:
=abs((((A2*B2)-SQRT((A2*B2)*(C2-A2)*(C2-B2)))/(A2*B2*C2))*60000000)

The result was a surprising 41 microseconds, far better than my previous result, and a pleasant surprise. 440 microseconds is the dead time for a 900 volt LND7313-1 pancake. The LND7317 is given as 80 microseconds, as is my Aware Electronics RM-80 using the 500 volt LND7313-3 GM tube.

So, not only is the SBT11A inexpensive and tiny, it is very efficient.
My window sill background readings with an SBT11A are averaging 15 to 16 CPM, while the larger LND7313-3 is averaging 30 CPM.