Dear all,
I have one question.
When neutrons bombard into Al target, what are possible products?
I did an experiment with Ge spectrometer, and got a spectrum has energy peaks: 2242 keV, 1731 keV, 1368 keV, 846 keV, 510 keV.
When I looked up the internet, I found the possible reactions can be: (http://www.geneseo.edu/nuclear/gamma-ray-spectra)
1. 27Al(n,gamma)28Al 2. 27Al(n,alpha)24Na 3. 27Al(n,p)27Mg
With this reference, I can find energy peaks of 24Na (1368 keV) and 27Mg (846 keV), the other peaks can be the coincidence of these peaks and also annihilation gamma rays.
However, the big question to me is: Where is the peak 1778 keV of 28Al (the cross section is large: 12 barns) ? I cannot find it on my spectrum. Is it something wrong?
In the reference link above, they said that the 1778 keV cannot be detected by HPGe spectrometer but can be detected with NaI spectrometer? Why is it ???
Many thanks for your help.
Related to the previous answer - what is your source of neutrons?
also what was the decay time between the end of irradiation/bombardment and counting? Given the 2.24 min half-life of 28Al if your decay time was much greater than 30-40 mins the vast majority of the 28Al would have decayed hence possibly explaining the absence of the 1779 keV 28Al photo peak.
Your three reactions (n,g), (n,p), and (n, alpha) are those you would expect. You photo peaks at 2242, 1731 (and 511) keV are due to 24Na - the presence of which is demonstrated by the 1368 keV photo peak. I suspect your Ge energy range is less than 0 - 2754 keV. The higher energy 24Na photo peak at 2754 will produce a single and double escape peak at 223
Don't know why but only part of my response was 'uploaded' previously.
To continue the higher energy 2754 keV photopeak will produce single and double escape peaks at 2243 and 1732 keV, respectively (your energy calibration might be off a keV or so) and also produces a 511 keV ann' peak. You noted a peak at 846 keV and attributed it to 27Mg. 56Mn emits a photopeak at 846 keV. The main photopeak of 27Mg is at 843 keV, to confirm 27Mg (quite probable) look for the next most intense peak of 27Mg at 1014 keV (if 56Mn then you should see a peak at 1810 and possibly at 2113 keV).
As noted earlier to properly answer your question one would need to know:
a) the neutron source (e.g., nuclear reactor, D-T, D-D n-generator, isotopic n-source, etc) and
b) the decay time between irr'n and the start of counting (also how long did you count the irradiated Al?)
Finally, off the top of my head the cross-section you listed for the 27Al(n,g)28Al reaction (12 barns) seems rather excessive - I'd check that too.
Hope this helps.
I fully agree withe MJM Duke. From my tables, the cross section of 27Al(n,g)28Al is rather 0.23 barn for thermal neutrons.
Yes the thermal neutron capture cross section for 27Al is indeed just 0,23 barn. So the only answer is, in the HPGe setup your n source just delivers no or only a very small amount of thermal neutrons. It´s no matter of detector. We need info about your applied n- source. Please read again the instruction you appended to your question.
As already mentioned by all the people above, it is very important to look into the following points when you work in gamma spectrometry, particularly in HPGe detector. To answer you properly following information is necessary.
1. calibration is done properly taking sufficient points, particularly the concern is in the high energy range (above 1408 keV)
2. time of irradiation as well as source of irradiation and its flux.
3. time of first gamma spectroscopic measurement
4. distance between source and the detector (detector geometry)
5. whether you collected time resolved spectra!
If you provide all the details then only it is possible to provide the clue!
Thank you for all of your replies.
1. Maybe the reference for 27Al(n,g)28Al is wrong. Can you please suggest for me a good reference? If it is just 0.23barn, I think this is reasonable.
2. In the experiment, the neutron source was the product of accelerated proton (80 MeV) with Be target. I forgot to count the time of irradiation. I will check in the logbook again. This is so so nice point. I also ask the question without notice the half-life of Al-28.
These answers are really helpful. I will check again my data.
I really appreciate for these !
@Bui,
I append you a data set (it´s old but correct) with thermal cross sections. Use sigma(0).
The neutrons produced by high energy protons (80 MeV) on Be-9 are very fast high energetic neutrons, which show without moderation nearly no thermal component, so the cited missing gamma component is caused by the missing reactions cross section as stated before.
Dear Krieger,
Actually, the neutron, which was produced be proton and Be target, was slowed down by water to become thermal neutron.
Thank you so much for your file. I will review mine.
A more general suggestion: If you go to: http://www.nndc.bnl.gov/exfor/exfor.htm you will get to the National Nuclear Data Center at Brookhaven National Laboratory. There you can find a lot of measured data for the various iterations of radiation/particles with matter. You will probably not find data that was measured/published in the last 12 months, but you will find a lot of historical data.
I fully agree with the answer given by Dr Duke from Alberta University. Also the Al(n,g) thermal cross section is small being 0.23 b as noted by others. Just wanted to add that the 24Na isotope produced by the 27Al(n,alpha) source may be used for producing high intensity 2754 keV gamma source. Such a source may also be produced in a nuclear reactor via the 23Na(n,g) reaction. In one application in using such a gamma source, the absolute cross section of the D(g,n) reaction was measured. See e.g. [Phys. Rev., C 39 (1989) 1247].
Raymond Moreh, Ben-Gurion University, Beer-Sheva, Israel
Considering the 12 barn question: http://www.osti.gov/scitech/servlets/purl/762101