Gamma Spectrometry Forum

Out there in Internetland I have found the half-life of Cs-137 being 30.15, 30.17, and 30.19 years. Why is there a range of values? And, just checking, I'm assuming that's 365 days per year, 24 hours per day, nothing to do with leap years, sidereal years, or other astronomy things.

Not that it really matters. It changes one or two of my range distances by a millimeter. But this is bugging me.

A value which i've seen used quite often is 30.05 years (+/- 0.08). This is a mean value...considering the random nature of radioactivity you are never going to get the same results twice if you measure such long half-life so a mean value is used instead.
When you are dealing with an isotope with half-life of 30 years, I am sure that there aren't that many cases when you need to know the half-life an accuracy of a few days.

I would like to know why isotopes decay ?

Reason tells me, if nothing is changing in our world then nothing should decay, so what causes nuclear decay ?

Let's agree to avoid vague terms like entropy 😂

That's something I don't see much outside of scientific literature -- error bars. I see these slightly different values for the Cs-137 half-life, but none of them with an uncertainty attached to them. In the pharmacy I see placards labeled 4.1365 GBq and I think, no it's not.

The professor once did a review of neutron lifetimes, and had to throw out half of them because it wasn't clear how the uncertainties were determined, therefore they couldn't be combined with the other measurements into an average. (And also because of "in-breeding", as he called it -- one results whose analysis depends on another whose interpretation just wasn't clear.)

The National Nuclear Data Center at the Brookhaven National Lab is a good resource for nuclear data.
https://www.nndc.bnl.gov/

There is a link that describes an on-going decay data evaluation project.
http://www.nucleide.org/DDEP.htm

One can search the NNDC's NuDat database -
https://www.nndc.bnl.gov/nudat3/

Results for Cs-137 follow -

Dataset #1:

Authors: E. Browne, J. K. Tuli Citation:Nuclear Data Sheets 108,2173 (2007)

Parent
Nucleus Parent
E(level) Parent
Jπ Parent
T1/2 Decay Mode GS-GS Q-value
(keV) Daughter
Nucleus Decay
Scheme ENSDF
file
137
55 Cs
0.0 7/2+ 30.08 y 9 β-: 100 % 1175.63 17
137
56 Ba

Beta-:

Energy
(keV) End-point energy
(keV) Intensity
(%) Dose
( MeV/Bq-s )
174.32 6 513.97 17 94.70 % 20 0.1651 4
334.65 8 892.13 20 5.8E-4 % 8 1.9E-6 3
416.26 8 1175.63 17 5.30 % 20 0.0221 8

Mean beta- energy: 187.1 keV 10, total beta- intensity: 100.0 % 3, mean beta- dose: 0.1871 MeV/Bq-s 12

Electrons:

Energy
(keV) Intensity
(%) Dose
( MeV/Bq-s )
Auger L 3.67 7.40 % 10 2.72E-4 4
Auger K 26.4 0.78 % 3 2.06E-4 9
CE K 624.216 3 7.79 % 11 0.0486 7
CE L 655.668 3 1.402 % 20 0.00920 13
CE M 660.364 3 0.300 % 4 0.00198 3
CE N 661.404 3 0.0646 % 9 4.27E-4 6
CE O 661.637 3 0.00965 % 14 6.39E-5 9

Gamma and X-ray radiation:

Energy
(keV) Intensity
(%) Dose
( MeV/Bq-s )
XR l 4.47 0.91 % 4 4.09E-5 19
XR kα2 31.817 1.99 % 5 6.34E-4 17
XR kα1 32.194 3.64 % 10 0.00117 3
XR kβ3 36.304 0.348 % 9 1.26E-4 3
XR kβ1 36.378 0.672 % 18 2.44E-4 6
XR kβ2 37.255 0.213 % 6 7.92E-5 21
283.5 1 5.8E-4 % 8 1.64E-6 23
661.657 3 85.10 % 20 0.5631 13

Wow. I knew individual teams produce new measurements of things now and then, but I didn't realize it was an international project! Looks like it's 30.08 years now, my working number is almost 0.3% too high! Thanks, Rob!