Experimental insights: Nuclear Orientation Thermometry

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 🙂

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7 thoughts on “Experimental insights: Nuclear Orientation Thermometry

  1. rrtucci says:

    The red thingie must be the NAI crystal and photomultiplier tube. Where is the NO thermometer and supercooled sample in this picture? What exactly is in the cardboard box? Thoroughly confused

  2. physicsandcake says:

    Ahh sorry, yes that is a bit confusing. Here we were just testing the (source) crystal at room temperature to see if we could see the gamma peaks, at this point it hadn’t been installed in the fridge. The cardboard box is the packaging in which the source was delivered. We left it in there so we didn’t have to handle it unnecessarily, and also so we wouldn’t lose it. It’s a pretty small crystal, and losing radioactive material doesn’t look good on the health and safety report 😉

    • chris busby says:

      Hi,

      I have the same set up i.e. cobalt 60 crystal and detector. I have taken warm counts and cold counts.

      Do you have any data on converting this into mK?

      Kind regards
      Chris

  3. emiro diez says:

    muy bien … excelente

  4. philippe says:

    What if you set the sample in a magnetic field (after shielding the PM) ? Maybe more spins would be aligned, and by decreasing random variations this would make NO thermometer useable at higher temp ?

  5. philippe says:

    My suggestion above is wrong, forget it.
    However, I do have a question: You say the spin directions align. But relative to what ? Is it related to the Co crystal structure ?

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