Here's a new one. See below for pics and text.
List of photos:
1.Gamma spectrum of Br-80 activity
2.“Spa care” product being turned into activation target
3.Plastic neutron hearth for Carl's Jr.
4.Scintillation detector and shielding
5.HVPS meters, showing 82 kV at 10.8 mA
6.Portrait of discharge at above conditions
7.Ion chamber (with a piece of 1/4” lead sheet taped to the back, reading 200+ mR / hr., much from neutron-induced gamma radiation!
Both natural isotopes of bromine have high neutron capture cross-sections, leading to the production of radioactive Br-80m, Br-80, Br-82m, and Br-82. See the following cross-section table:
Thermal cross-sections
Br-79 - > Br-80m (4.42 hr): 2.5 b
Br-79 - > Br-80 (17.7 min): 8.3 b
Br-81 - > Br-82m (6.1 min): 2.4 b
Br-81 - > Br-82 (1.47 day): 0.25 b
Resonance integrals
Br-79 - > Br-80m (4.42 hr): 30 b
Br-79 - > Br-80 (17.7 min): 95 b
Br-81 - > Br-82m (6.1 min): 51 b
Clearly, production of Br-80 by epithermal (resonance) neutrons is the predominant cross-section of interest. Production of Br-80 is also the highest cross-section for thermal neutrons. Some Br-80 will also be produced by the long decay of parent Br-80m. Br-80 is a weak positron emitter, so it can be detected by gamma spectroscopy by virtue of the annihilation radiation produced by its positrons. It also undergoes normal negative beta decay, producing a 616-keV gamma ray in about 7% of decays. Br-82m produces essentially no gamma rays, unfortunately; Br-82 kicks out various gamma rays with high intensity, particularly 777 keV. Unfortunately, its half-life is quite long. I do not yet know if I have detectable Br-82, because I am still counting the sample.
For this experiment, bromine was obtained at my Lowe's hardware store. It is the beginning of the summer, and so the pool-care section is fully stocked. Pure sodium bromide (NaBr) is sold in little 2-oz. bags intended for use in cleaning spas. It is called “Spa Start-Up” and is made by Du Pont (see photo). I made an activation target by filling a little rectangular plastic box with the contents of one entire package of Spa Start-Up. This box slides tightly into my detector shield (see photo of detector). The box was placed into the polyethylene “neutron hearth” that is now erected alongside Carl's Jr. The activation region is behind 2 inches of UHMW PE from the fusor surface, and is backed by an additional 6+ inches on all other sides.
This activation run lasted 20 minutes with Carl's Jr. operating at 10 mTorr, 80-85 kV, and 10-11 mA. I had originally intended to run for 40 minutes, but at 17 minutes in, my current-monitor VTVM was violently destroyed and took out the circuit breaker. I finished out 20 minutes with everything but the HVPS running on my backup inverter.
Then the sample was counted with a 1” NaI(Tl) detector. After subtracting a subsequent background, the graph below results. The peak between 615-620 keV is pretty clearly Br-80. The peak below that, lining up at about 500 keV, is probably the annihilation-radiation peak for Br-80. No peaks associated with Br-82 or Na-24 are in evidence, but this was a short count. A longer one is now in progress.
I need to upgrade my spectroscopy setup badly! I keep around my 1x1.5” NaI(Tl) because of its 6.8% resolution, but it does not have the kind of volume I really need for doing these experiments. I also live in one of the most (THE most?) radioactive domiciles in the state of New Mexico on account of my rad collection, so my background is ungodly.
-Carl
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2007-05-21 01:08 Neutron activation of bromine 
(Carl Willis) [Latest: 2007-05-22 23:38]