For many cosmogenic nuclides, production from muons represents a small but important component, especially if the sample is extracted at depth. The production pathways for each nuclide type shipped with ACE are shown below:
|
Nuclide |
Spallation |
Muons |
Low-Energy Neutron Capture |
| 3He | √ | ||
| 10Be | √ | √ | |
| 14C | √ | √ | |
| 21Ne | √ | ||
| 26Al | √ | √ | |
| 36Cl | √ | √ | √ |
The only default-supplied nuclide for which low-energy neutron capture is considered is 36Cl. The procedure by which ACE calibrates and dates using low-energy neutron capture is documented in a previous article.
Muons production rates are not calibrated for in ACE, as compared to spallation they can generally be considered second order. During ACE’s experimental design phase muon production rates are requested as inputs to calibration:
In the above example, muon production rates are given as 2% of total production for both fast and slow muons, a common choice for 10Be and 26Al. For 36Cl, muon production rates are input as muon flux rates (muons cm-2 yr-1), with 175 and 700000 the default values for slow and fast, respectively.
In a calibration, ACE takes these muon calibration rates, determines the component of the nuclide inventory which can be attributed to muon production and subtracts it from the total. In this way, HLSL spallation and low-energy neutron capture production rates will be directly proportional to the inventory and HLSL rates can be estimated via linear regression. However, muon production rates scale differently to spallation production rates, and these must be accounted for during dating and calibration.
So how does ACE treat muons? Essentially just as described above. During calibration, muon contributions are removed according to the percentage and scalings chosen, and during dating post-calibrated percentages and scalings are used. Something to be aware of is that during the calibration, ACE doesn’t know what the total production rates are, so cannot tell if the percentages of total are accurate. In the above experiment 2% contributions from fast and slow muons were input. However, post calibration the muon contributions will look slightly different:
Click on Image to Expand
While the pre-calibrated muon production rates were entered as 2%, the post-calibrated production rates are 1.91%. While this is unfortunate, the the total rates are known reasonably well in advance so differences pre- and post- calibration should be small. If you really need exact numbers for post-calibrated muon contributions, try making a new experiment and changing the inputs slightly. For example if 2% was required post-calibration, try 2.1% pre-calibration and iterate from there.
Generally, muon contributions are small and pre- and post-calibration differences should be almost inconsequential. To see some examples, take a look at the muon production rate profiles with depth on the Production Rate Profile utility.