Alan has a full set of gamma check sources. Initial calibration was done with 137Cs. The energy scale drifted over time with heating from the reactor. The background always showed the 78keV x-ray and 1461 40K background peaks. I re-calibrated the energy scale on every file, resampled each to 1keV/bin, and insured that the resampled file was adjusted to have the same photometric counts. Once that was done I could subtract my calibrated background.
We have since run another calibration with the equipment still in place. We setup to simultaneously capture spectra files with the NaI scintillator - spectrometer, and also with the GMC-320+ GM counter he had alongside the reactor. He placed the check sources 1 by 1 on top of the reactor tube and captured the response to each. These sources were used: 54Mn, 133Ba, 57Co, 109Cd, and 137Cs. There is the capability to capture the time series of counts out the back of the spectrometer, but Alan did not have anything to capture it. We will get that setup for next time. He also didn't know that the software had provision for automated successive integrations and captures. Now I have told him, and that won't be manual and irregular next time. The background was quite constant - the radon was either a minor factor or was essentially constant over the course of the integrations. Finlay McNab wrote me and asked about possible cosmic ray shower. It would be good to have another detector placed away from the reaction, but I think that was covered and here is what I told him about why I don't think it was a cosmic ray shower: "The NaI detector was in a cave of lead bricks 3" thick. At 500keV, only 1ppm of incident cosmic ray energy will penetrate. By 1MeV, 0.2% will penetrate. So when high energy cosmic rays hit the lead cave, some will penetrate and when they interact with the lead, and the 78keV characteristic x-ray will be generated. However, this will not go very far in the lead. The very high energy cosmic rays that penetrate almost all the way through the lead and excite the 78keV x-ray near the inside surface will be picked up in the NaI detector - and we see this. What we see is that this 78keV peak and the rest of the background stayed photometrically stable. When we subtract the reference background from other traces with no signal, we just get zero mean noise. If the cosmic rays had peaked, it would have peaked the 78keV signal and this would no longer have subtracted out. We see a clean subtraction in our Spectrum-07 with no evidence of the 78keV peak, so it is reasonable to conclude that the cosmic rays did not have a sudden shower." I don't think trying to measure the radon daughters is worthwhile. They would have to be checked with a beta detector anyway, not with the NaI because there is little or no gamma from radon decay as I understand it. Bob On Wed, Feb 24, 2016 at 3:35 PM, Russ George <russ.geo...@gmail.com> wrote: > Was there some sort of calibration with some known radiation sources > performed with the same NaI instrument in the lab setting. Say placing a > Coleman lantern mantle, thorium laced, for a reading, or a banana or cupful > of Salt Substitute KCl…. Plenty of known ‘reference radiation sources’ are > easily within reach of the local Walmart or grocery store. Just to make > sure the instrument was performing as expected? > > > > How about the time series of the counts, hopefully the counts were binned > in many files and not a single lumped file. > > > > Any insight on the instrument and its performance would be very useful. > > > > If Santa Cruz is as reported a high radon area then a simple filter > collection will provide plenty of ‘radon fleas’ to study with the > instrument. Quick and dirty - place a paper coffee filter over the end of a > vacuum cleaner hose, run the vacuum for a time – say half an hour, stir up > the dust in the room by sweeping the floor with a broom… examine the filter > with the instrument. A longer slower collection seeking ‘radon fleas’ is > easily accomplished with a computer CPU fan, box it in duct tape, apply the > paper coffee filter, run for a few days, examine filter for flea signature. >
