Statistical errors in residual dose rates and activities and number of primaries/batches

Hello everyone,

I was recently wondering about how to reduce the statistical fluctuations in residual dose rate calculations and thought
that if I increase number or primaries per run for a total number of primaries, I would see a reduction in statistical
errors but this is not what I see. As I understand it, the values from different batches give the variance of the mean
for a given quantity.

In the study I had two sets of runs, each totalling 100 M primaries.
First set: 1 M primaries * 100 batches
Second set: 10 M primaries * 10 batches

Residual dose rate results:

   Time Tot. response (pSv/s)
               First set Second set
--------------------------------------------------------------------------
   1h_SOB 7.3209280E+07 +/- 0.2788494 % 7.3209280E+07 +/- 0.1696288 %
   1d_SOB 3.9039216E+08 +/- 0.1965424 % 3.9039216E+08 +/- 0.1532526 %
   3d_SOB 4.5325405E+08 +/- 0.1940250 % 4.5325405E+08 +/- 0.1533875 %
10d_SOB 4.6606554E+08 +/- 0.1911391 % 4.6606554E+08 +/- 0.1504155 %
   0s_EOB 4.7146954E+08 +/- 0.1893668 % 4.7146954E+08 +/- 0.1492323 %
   1h_EOB 4.0237386E+08 +/- 0.2029038 % 4.0237386E+08 +/- 0.1707704 %
   1d_EOB 7.9427664E+07 +/- 0.3225139 % 7.9427664E+07 +/- 0.3200963 %
10d_EOB 6036197. +/- 0.3228723 % 6036197. +/- 0.3079922 %
40d_EOB 190254. +/- 0.2063927 % 2190254. +/- 0.1794188 %
   1y_EOB 358294.0 +/- 0.3270685 % 358294.0 +/- 0.3348824 %
   2y_EOB 214956.4 +/- 0.3732046 % 214956.4 +/- 0.3926361 %
   3y_EOB 147807.1 +/- 0.3735141 % 147807.1 +/- 0.3969630 %
   5y_EOB 71939.66 +/- 0.3682632 % 71939.66 +/- 0.4093317 %

Why do I not see a reduction in the variance of the mean in the second set? When I look at the dominant activities in
different regions (1y, 3y, and 5y after EOB), I don't see much difference in the errors between each set. So doesn't
seem to be advantageous to run a larger set of primaries per batch.

How are the errors in activities determined? Looking at production rates and time evolution of activities, seems as if
the errors are determined from production rates and remain the same for most but not all isotopes (see Lu,172 below).
This might have to do with the grow-in contribution from decay of other isotopes.

Also, sometimes I see larger errors for smaller activities (on the order of 10^2 Bq) that for larger activities (on the
order of 10^4 Bq). So I don't think the fluctuations in activities depend on the production rates alone.

Thanks and best wishes,
Mina

Here, I am listing production rates and activities in a Tantalum target at different times after End of Beam a subset of
isotopes:

Production rates (nuclei/prim):
Mn,56,25, 9.7495E-22 +/- 66.68 %
Co,60,27, 4.4082E+04 +/- 99.00 %
Se,75,34, 8.1350E+04 +/- 99.00 %
Y, 88,39, 2.6066E+04 +/- 55.28 %
Lu,173,71, 3.6126E+08 +/- 0.24 %
Lu,172,71, 1.8536E+09 +/- 2.110 %
Hf,175,72 1.6301E+10 +/- 0.09%
Hf,172,72, 4.8303E+08 +/- 0.3%

Activities (Bq):

0 sec after EOB:
Mn,56,25, 9.99e+06 +/- 66.67 %
Co,60,27, 4.47e+04 +/- 99.00 %
Se,75,34, 6.73e+05 +/- 99.00 %
Y, 88,39, 3.00e+06 +/- 55.28 %
Lu,173,71, 3.90e+10 +/- 0.24 %
Lu,172,71, 2.53e+10 +/- 1.46 %
Hf,175,72, 7.21e+11 +/- 0.09 %
Hf,172,72, 1.23e+10 +/- 0.30 %

10 days after EOB:
Mn,56,25, 9.75e-22 +/- 66.68 %
Co,60,27, 4.46e+04 +/- 99.00 %
Se,75,34, 6.35e+05 +/- 99.00 %
Y,88,39, 2.81e+06 +/- 55.28 %
Lu,173,71, 4.11e+10 +/- 0.24 %
Lu,172,71, 1.22e+10 +/- 1.07 %
Hf,175,72, 6.73e+11 +/- 0.09 %
Hf,172,72, 1.22e+10 +/- 0.30 %

40 days after EOB:
Co,60,27, 4.41e+04 +/- 99.00 %
Se,75,34, 5.34e+05 +/- 99.00 %
Y,88,39, 2.31e+06 +/- 55.28 %
Lu,173,71, 3.94e+10 +/- 0.24 %
Lu,172,71, 5.15e+09 +/- 0.29 %
Hf,175,72, 5.00e+11 +/- 0.09 %
Hf,172,72, 1.19e+10 +/- 0.30 %

1 year after EOB:
Co,60,27, 3.92e+04 +/- 99.00 %
Se,75,34, 8.14e+04 +/- 99.00 %
Y,88,39, 2.79e+05 +/- 55.28 %
Lu,173,71, 2.51e+10 +/- 0.24 %
Lu,172,71, 3.47e+09 +/- 0.30 %
Hf,175,72, 2.00e+10 +/- 0.09 %
Hf,172,72, 8.52e+09 +/- 0.30 %

3 years after EOB:
Co,60,27, 3.01e+04 +/- 99.00 %
Se,75,34, 1.19e+03 +/- 99.00 %
Y,88,39, 2.43e+03 +/- 55.28 %
Lu,173,71, 9.14e+09 +/- 0.24 %
Lu,172,71, 1.65e+09 +/- 0.30 %
Hf,175,72, 1.45e+07 +/- 0.09 %
Hf,172,72, 4.06e+09 +/- 0.30 %

5 years after EOB:
Co,60,27, 2.32e+04 +/- 99.00 %
Se,75,34, 1.74e+01 +/- 99.00 %
Y,88,39, 2.11e+01 +/- 55.28 %
Lu,173,71, 3.32e+09 +/- 0.24 %
Lu,172,71, 7.87e+08 +/- 0.30 %
Hf,175,72, 1.05e+04 +/- 0.09 %
Hf,172,72, 1.93e+09 +/- 0.30 %
Received on Sat Aug 11 2012 - 11:49:30 CEST