Part of the reason I started this blog was to disseminate information about what experimental physicists actually do in the lab on a day-to-day basis, and why these things are interesting. The posts should be accessible to a wide audience. Here’s the first one…
Experimental insights: Nuclear Orientation (NO) Thermometry
I spent some of last week playing with a NO thermometer. This consists of a radioactive source crystal, here a single crystal of Co with 60-Co impurities, which is mounted (in our case) on the low temerature stage of the dilution refrigerator. The 60-Co decays to the stable isotope 60-Ni via beta decay and the emission of two gamma rays at 1.17 and 1.33 MeV. The gamma rays can be detected by (for example) an NaI scintillation detector.
What does this have to do with temperature? Whilst the decay itself is temperature independent (you can’t define the temperature of a nucleus!) the gamma rays are emitted in a specific direction – along the spin axis of the nucleus. As the temperature decreases, the 60-Co nuclei become polarized (their spin directions align, or more accurately they are no longer misaligned by thermal fluctuations), and the spatial distribution of emission develops nodes along one crystal axis direction (see sketch below). So point your detector along this axis and you can watch the counts fall to zero as you lower the temperature.
Here is a picture of the detector setup. The source is in the cardboard box, and the software displays the energy spectrum of detected particles. (You can see the Compton scattering continuum and the two gamma peaks to the right).
To calculate the temperature you take the ratio of the integral under the peaks at high temperature to that at low temperature. The NO is an absolute temperature reference, i.e. you calibrate it from comparison with the theoretical prediction for the decay rate.
The thermometer only really works below 100mK, above this temperature nearly all of the nuclei are randomized. It works best in the range ~5-50mK.
The NO is usually removed in normal operation, as the radiation can cause a slight heating of the sample. It is used to check the base temperature of the fridge and to calibrate other (nearby) thermometers. At the moment there are 7 thermometers in total on our big dilution fridge – 4 of them are on the mixing chamber where the sample sits. We’ll see if it works 🙂