Measurement of excited Np237 half-life via the alpha-gamma coincidence method.

Measurement of excited Np237 half-life via the alpha-gamma coincidence method.

Author: Tim

241Am is a favorite source for nuclear scientists, as it is readily available for experimentation with geiger counters, cloud chambers, spectroscopy and the like. 241Am decays 100% via alpha emission to 237Np, approximately 85% of those decays are accompanied with a 59.54keV gamma ray. The origin of that gamma is not the 241Am, but rather the decay (“relaxation”) of an excited state of the 237Np nucleus, denoted here as 237Npm. The 237Npm decays quickly to the ground state of 237Np, but not too quickly. The following experiment measures the half-life to be 68ns which is in agreement with the accepted value of 67ns.

Click on this decay scheme image to go to the LNHB complete Am-241 decay scheme.

To make this measurement, both alpha and gamma detectors were needed, as well as the NIM electronics to detect the alpha-gamma coincidence. I had it mostly easy, being able to draw everything needed from my surplus NIM module collection, however, as with any surplus NIM module collection, several modules did need repair. Also, I used almost every BNC-BNC cable in my shop!

Because alpha particles have a very short range in air, the alpha particle detection had to occur in a vacuum. I put together this a small vacuum chamber, only pumped on with a direct-drive mechanical pump, to reach 10mTorr. However, the 59.5keV gamma rays that also need to be detected are no match for the solid stainless steel wall of the vacuum chamber. Luckily, I still have my home-made 0.007-inch thick Be vacuum window mounted to a 2.75 inch ConFlat flange. I made this for my “x-rays from scotch tape” experiment about a decade ago. The gamma ray detector, a NaI(Tl) scintillator was placed close to the Be window (the widow has a 1-inch diameter active area). The NaI(Tl) detector is shielded by a portable bench top thyroid scan lead shield.

Top view of the vacuum chamber and detector arrangement. The alpha particle detector resides in the stainless steel tube that is at the “7:30” (bottom left) position. The NaI(Tl) gamma ray detector is inside the tan-colored lead shield sitting at the “9 O’Clock” position.
Carefully handing the silicon barrier alpha particle detector. The active portion of the detector is the entire surface of the recessed region.
The BNC-vacuum feed through to BNC barrel –> Microdot adapter –> Si alpha detector stalk.
Side view of the experimental setup, showing the Ortec 142 preamp connected to the alpha detector through the BNC feedthrough in the foreground.
The approximate placement of the NaI(Tl) detector inside the bench top lead shield.
Another side view of the setup showing connections to the preamplifier/PMT base, but also the Ortec 113 preamplifier connected directly to the PMT anode signal – this is used for fast timing.
The overall detector system, timing electronics, and data acquistion.
This schematic describes the setup of the previous photograph.
Before we begin the alpha-gamma coincidence data acquisition we need to calibrate the time-to-amplitude-coverter, or simply the TAC. This was accomplished by using a two-channel Berkeley Nucleonics 555 adjustable pulse and delay generator, and monitored by an oscilloscope of course. Note yellow and blue traces on the oscilloscope on the right, and the delay between them. Also note the spikes on the computer monitor in the top left – they are space 100ns apart.
I took nine calibration points, each spaced by 100ns, and recorded the peak MCA channel for each “Delta-T marker” location and performed a linear fit, shown here. Now we have a MCA channel-to-time calibration.
First few coincidences!!! The yellow trace on the oscilloscope is the alpha detector signal, the blue trace, following shortly thereafter, is the gamma ray detector signal, and the green trace is a logic “high” signal to provide a gate for a future experiment. Now it is all working, let it collect data for about a day…
The half-life can be established by fitting an exponential decay curve to the data. Because of the TAC’s limitation at small time intervals, the fit begins at 100ns. The fit returns a decay half-life value of 68ns, in good agreement with the established 97ns!