Uranium is not an easy nut to crack--its overwhelming natural radioactivity buries the signature radiations of most induced activities one might potentially create by exposure to a weak, continuous neutron source. Jon R. beat me to the attempt, trying to detect the production of U-239 in DU with his superlative HPGe system. He reported that the expected low-energy gamma peak from U-239 could not be discerned for all the x-ray and downscattered gamma noise from the DU's natural activity.
I am now focusing instead on detecting energetic (>1 MeV) gamma rays from short-lived fission products. The highest-energy major gamma ray in the short-term uranium decay series is 1.01 MeV from Pa-234m. By restricting my region-of-interest to radiations above this level (1-5 MeV), I am hoping to capitalize on a better signal-to-noise ratio.
For this sequence of experiments a quantity of fresh natural uranium was produced. About 5 lb of "schlock rock," non-collector-grade Utah uraninite in dolomitic / calciferous sandstone, was used as the feedstock. The uranium was leached with muriatic acid, precipitated from the leachate with all other alkaline-insolubles using ammonia, and isolated and purified with a quite-selective carbonate / peroxide cycle. I attempted to calcine the final hydrous uranyl peroxide "yellowcake" to anhydrous UO3 at 200 C, but the finished material was found to still contain hints of uranyl peroxide despite attaining the proper Fiesta-red color. Bottom line is that the resultant material from this process should be considered unsafe in contact with any oxidizable organic solvents. Anyway, the yield from this batch was 70 g of UO3. I will make an illustrated post describing the uranium chemistry a little later.
I started making irradiations of the uranium almost as soon as it was out of the oven, but did not really have my ducks in a row for about a week. Unfortunately, Th-234 and Pa-234m build in quickly, and so my UO3 is now much more radioactive than it was when fresh.
Three irradiations are presented in the graphs at bottom. First, I irradiated about 20g of UO3 in contact with water, then decanted the supernatant water to count for fission product activity > 1 MeV. I used a shielded 2x2" NaI(Tl) scintillator in conjunction with a multichannel scaler and set the LLD just above the 1.0 MeV Pa-234m peak. I found that the water does have above-background activity that decays with a period of 15-25 minutes after irradiation in my plastic "flux trap". (See Graph 1.) Next, concerned that the activity in the water might be due to traces of natural decay products like Bi-214, I irradiated a dry sample of UO3. I interposed 1/8" of lead in front of the NaI detector to cut down all energetic beta particles that otherwise could impact the detector. Despite the increased pileup background from having the uranium itself in front of the detector and the fact that my fusor's grid had failed on the run, short-lived decaying activity can be discerned (second graph). Last night I got another run in, having replaced my small grid with the old big one. For this run I returned to the low-background wet technique, leaving the lead beta shield in place. Results are shown in the last graph. Despite high uncertainty, the first and last runs have sufficient quality of data to make some least-squares decay-curve fits. The observed half-life is on the order of 19 min., though the uncertainties are large. My guess is it represents the additive contributions of several prominent water-soluble fission products, including perhaps Rb-88 and Rb-89. Time now to set up the MCA to look for signature high-energy peaks.
More later...
-Carl
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2007-09-10 19:38 Neutron Activation of Uranium: Early Results 
(Carl Willis) [Latest: 2007-09-18 19:28]