Beta decay of 11Be

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_Geant4 Version: geant4-10-07-patch-04
_Operating System: Rocky linux 8
_Compiler/Version:gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-18)
_CMake Version: cmake version 3.20.2

Hello experts,

I’m using Geant4 to simulate the response of an array of NaI(Tl) detectors covering roughly 95% of the solid angle around a target position. The calculations made for the 137Cs , 22Na and 60Co sources agree very well with my experimental measurements. Now I’m simulating the beta decay of 11Be, 11Be(beta-, gamma)11B, and comparing the simulated results with the experiment.

In the experiment, an 11Be beam is implanted on the Silicon detector placed at the center of the NaI array. We accumulate 11Be for 30s and measure the gammas (NaI) in coincidence with the betas (Silicon detector) in the following 30 s. I replicated the geometry of the experiment in Geant4.

I included in my physics list:





I place the 11Be ions at the silicon position

.mac file


/gun/particle ion

/gun/ion 4 11

/gun/energy 0.0 MeV

/grdm/nucleusLimits 11 11 4 5

/run/printProgress 10000

/run/beamOn 100000


The spectrum looks similar to what was expected, but there is an extra gamma peak at around 10 MeV that is not supposed to be there. Do you have any explanation for this feature?
Also, I’m checking the particle ID in the silicon detector and I find that it is 2 (positron) instead of 3 (electron)

Just using a ruler on your spectrum, I estimate that the high energy peak is at 9.871MeV. This is suspiciously close to the 9.870MeV level of 11B (Lederer and Shirley, Table of Isotopes 1978). That level is populated by the beta decay of 11Be 3% of the time. (It can be up to 15% if the level alpha decays to 7Li. We’ll forget about that mode, since its Q-value precludes an energy deposit of 9.871MeV.) In populating the 9.870MeV level, the beta has an energy of ~1.64MeV, since the 11Be->beta->11B Q-value is 11.509MeV.

So if the beta escapes the detector and the excited 11B ion can deexcite via any means other than alpha decay, it will deposit 9.870MeV in the detector.

The only problem is that I cannot find any gamma cascade from that level to the ground state.

" The only problem is that I cannot find any gamma cascade from that level to the ground state. "

But Geant4 does ! (it should not ? …)
Here, a macro for rdecay01 and its printout.

marina2.mac.txt (313 Bytes)
marina2.out.txt (2.9 KB)

Hi Michel,

This is quite interesting. I tend to trust ‘Table of Isotopes’, even though that particular volume is old (unfortunately, the only one I have on hand). More modern online data, like the TUNL Nuclear Data Project, seem to agree with it, but that doesn’t mean they are correct. On the other hand, when I look at the level diagram, I cannot see any good reason (like being spin forbidden) that would prevent a gamma cascade to ground from the 9.870 level.

This has become interesting. I’ll have to dig a little further.

Hello, the problem is that there is no evidence in the data of any gamma transition above the one at 7.9 MeV.
I attach my sum spectrum for a fraction of the data. The red line is the experimental spectrum after background subtraction; the green line is the result of the geant4 simulation

Does your Geant4 (green) spectrum, or alternatively the experimental (red) spectrum, include a correction for detector efficiency, which should pretty small at 10MeV?

Hi, The green spectrum from Geant4 includes the broadening of the peaks calculated from the energy resolution as a function of the gamma energy measured for my detectors and an energy threshold set as in the experiment. The red spectrum (experiment) is not corrected for the efficiency. These are preliminary data, I need to double check the normalization. I just wanted to show that at around 10 MeV the experimental spectrum is quite empty. Also Millener et al. Physical Review C 26 (1982) 1167 suggest the 11B level at 9.9 MeV decays by alpha emission.

…red spectrum (experiment) is not corrected…I just wanted to show that at around 10 MeV the experimental spectrum is quite empty.

My point was only that if the experimental efficiency were very low, which I would expect for a Si detector for near 10 MeV gamma rays, it would be difficult to tell if there was a weak peak there or not.

Also Millener et al. Physical Review C 26 (1982) 1167 suggest the 11B level at 9.9 MeV decays by alpha emission.

I agree. Lederer and Shirley also show the alpha decay to 7Li as being the only channel depopulating the level. Likewise the TUNL data base. And I can find no paper reporting a ~9.78 MeV gamma deexitation of the level.

The Si detector is only for detecting the betas from the 11Be decay. In the experiment, we implanted the 11Be ions on a 1.5 mm thick Silicon detector and detected the emitted betas. We detected the gamma rays (in coincidence with the betas) with an array of 16 NaI(Tl) detectors with dimensions 2"x4"x16" each, closely packed around the silicon detector. From a rough estimate, the detection efficiency of the system at around 10 MeV should be only slightly less than that at 8 MeV. I believe we should have seen a sign of the 10 MeV transition.

Yes, I agree.

So Michel, do you think it is premature to submit this as a bug report?

It is not premature. Please, go on.

Please keep me posted on this subject and let me know if there is anything I can help with.

I have created Bugzilla problem 2577 ‘Beta decay of 11Be produces gamma ray not in experimental data’ I will keep you posted as to the outcome.

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For the moment, I solved the problem like this:

  1. I copied the file z4.a11 to my local working directory.
  2. I removed the last line corresponding to the 9.9 MeV state.
  3. I set my .mac file to read the decay data from this modified file /grdm/setRadioactiveDecayFile 4 11 z4.a11
    Now, data and simulation agree quite well.
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