How do you calibrate a neutrino detector?

Factual Questions
1

I’ve read recently about the solar neutrino problem, and my question is, how do they know that the detector isn’t simply undercounting neutrinos?

-Ben

2

Nuclear reactors give off neutrinos at a rate that can be calculated from the rate of the fission reation.
If you know the distance between your neutrino detector and a nuke plant you can calculate how many neutrinos per second arrive at your detector from the plant.
If you then damp the reactor and measure the number of neutrinos per second at your detector, the difference between the two measurements will allow you to calculate the sensitivity of your instrument.

3

Basically, you stick a nuclear reactor next to it. Every time an atom decays by beta decay (neutron turns into proton and emits an electron), or goes through a few other processes, it also emits a neutrino (strictly speaking, an antineutrino, for beta decay). We can easily measure the number of beta particles (electrons) produced, and we know that the same number of neutrinoes are produced.

There are problems, of course. For starters, we know that all neutrino detectors vastly undercount the neutrinoes they receive, because they’re so weakly interacting: It’s a matter of figuring out just how much they’re undercounting. Also, most detectors are only sensitive to one of the three flavors of neutrino (electron, mu, or tau), and only to a few specific energies. If the Sun were producing neutrinoes at some other energy, or if they were changing type between there and here, then we wouldn’t detect them. If the deficiency in solar neutrinoes was only observed at one detector, or at detectors of one type, then this would be the accepted explanation. However, all detectors, of any type yet devised, still show a deficiency. The data is only just starting to come in from the newest detectors, capable of finding all three flavors, but I don’t know if this points to the solution yet.

4

How do you position the reactor next to it? I had assumed they shot a beam of neutrinos at them from an accelerator. My question, then, would be how they know the difference between an underproducing sun and an overproducing accelerator. If they use a reactor, then I can see how the relationship between # of neutrinos and # of beta particles is a lot more certain than our understanding of the Sun.

-Ben

5

The reactor doesn’t have to be “next to it”. Since neutrinos interact so weakly with matter about the only thing that will affect the intensity of the signal (ie neutrinos per second at the detector) will be the inverse square law. You’ll get a bigger signal if the reactor is within a few hundred miles (as with the Soudan detector in Minnesota and Fermilab in Illinois), but there’s no physical reason you couldn’t calibrate against a reactor on the far side of the planet. -Of course as the distance goes up, you might start to see some interconversion between the various types of neutrino. If that were to happen, the solar neutrino problem might finally be solved.

6

This raises a few more questions:

  1. Super Kamiokande is, IIUC, set up to observe multiple neutrino types, and it is claimed that they may have observed some interconversions. Why do some detectors only detect certain kinds of neutrinos?

  2. A neutrino beam (at least one from an accelerator, with sufficient brightness) will show a detectable dimming after passing through the earth, and Leon Lederman has proposed a deep-earth tomography project based on this principle. Am I right in assuming that this loss is a lot less than the number of missing solar neutrinos?

  3. If you make a neutrino beam using an accelerator, how do you aim it?

-Ben

7
  1. A google search on the phrase “solar neutrino problem” turns up all kinds of information on detector design.
  2. The sun is only putting out ~half the expected number of neutrinos.
  3. You can’t steer neutrinos, so you have to change the geometry of the reaction that produces them to aim a beam.