So I'm working on a nuclear physics problem and am looking at radioactive decay. The common unit used for very long decays is years within the literature. Is this the sidereal or tropical year? I want to use units of seconds but seeing as how these 20 minutes 24.5 seconds that differential will add up over time...
I would guess tropical but that's just a guess.
And on the same note, what about days? 24 hours? or 23 hours 56 minutes and 4.1 seconds?
Bonus points for a source
A "year" without qualification may refer to a Julian year (of exactly $31\,557\,600~\rm s$), a mean Gregorian year (of exactly $31\,556\,952~\rm s$), an "ordinary" year (of exactly $31\,536\,000~\rm s$), or any number of other things (not all of which are quite so precisely defined).
Radioactive decay tables tend to be compiled from multiple different sources, most of which don't clarify which definition of "year" they used, so it is unclear what definition of year is used throughout. It's quite possible that many tables aren't even consistent with the definition of "year" used to calculate the decay times.
On the other hand, the standard error is usually overwhelmingly larger than the deviation created by using any common definition of year, so it doesn't really make a difference.
A day in physics without qualification pretty universally refers to a period of exactly $86\,400~\rm s$.
Years are merely an approximation, as you pointed out, they really aren't precisely defined. In physics seconds are used as they can be calculated exactly using atomic clocks.
For instance, no application requires an exact decimal representation of years, you can round to approximate numbers and then use a remainder of seconds.
As a not necessarily representative example, the decay data in the NUCLIDES 2000 database, which is based on the JEF2.2 decay data file, use a year of 365 days and a day of 24 hours.
For example, the halflife of Co-60 is internally stored as 1.6623E+08 seconds but reported as 5.2711E+00 years.
The decay data provided in
use a year of 365.2422 days and a day of 24 hours (which corresponds to a tropical year of 365 days, 5 hours, 48 minutes, and 46 seconds).
As stated in previous answers, the difference between the different year definitions is too small to matter unless you are dealing with very precise measurements. However, it seems that some people do have data precise enough to care about an exact definition of the year.
In astronomy, when the term year is used as a unit of time (rather than a variable astronomical period), it is understood as a Julian year of exactly 365.25 × 86400 SI seconds. This is the basis of the definition of the light-year.
This definition, however, does not necessarily apply to other disciplines. In 2011, the International Union of Pure and Applied Chemistry (IUPAC) and the International Union of Geological Sciences (IUGS) jointly recommended to define the year (called annus, symbol: a) as 3.1556925445e7 SI seconds, which is the length of the tropical year for the epoch 2000.0. The recommendation seemed to meet some resistance, so do not take for granted that everybody follows it.
My guess is that anyone publishing data precise enough for the definition to really matter will presumably specify the precise definition they are using.
