Benjamin Monreal's Home Page
Benjamin Monreal's Home Page |
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Neutrino detection
Stand outside on a sunny day; the top of your head has a surface area of about 0.1 square meters. Every second, your head will be hit by about 3x10^26 air molecules (total energy 1,000,000 Joules! Fortunately, you don't absorb all of this energy.), about 10^20 photons (total energy=50 J, which you do absorb), about 10 high-energy muons (total energy=6 nanojoules, of which your body might absorb 10%), and maybe 1 high-energy proton. That's sort of the order in which these particles were discovered: molecules by Dalton and Avogadro around 1810; photons by Einstein around 1905; cosmic-ray muons by Victor Hess in 1912.
You can detect the air and the light yourself; stop by your local university physics lab and, in about an hour's work, you can detect the muons and the protons on your own. There are also billions of neutrinos passing through your 1/10 m^2 head: 10^3 per second from the creation and decay of the aforementioned high-energy muons, 6x10^13 from nuclear reactions in the Sun, 6x10^3 from natural radioactive isotopes deep underground. Billions more if you live near a nuclear power plant, or even a nuclear naval base. Despite their huge numbers and fairly-large energies, these neutrinos are very hard to detect; there are only a handful of labs worldwide that can do it at all. If you can detect them---and, especially, if you can detect different types of neutrinos, from different sources, and after different flight-distances---you can derive interesting details like the neutrino mass and coupling. In more detail ...I'm active on the Sudbury Neutrino Observatory (SNO), a large heavy-water neutrino target 6400 feet underground in Ontario, Canada. SNO detects neutrinos in two ways: (any-flavor) nu + e -> nu + e scattering; detection of the electron from (electron-type only) nu_e + d -> p+p+e; and detection of the neutron from (any flavor) nu + d -> p + n + nu. The electrons were detected, in all cases, by Cerenkov radiation illuminating a spherical array of photomultiplier tubes. The neutrons were detected by one of two methods: by capture on 35Cl (dissolved in the water as NaCl), yielding about 8 MeV of gamma rays which are detected by the PMT array; or by capture on 3He in a submerged proportional counter tube, yielding the reaction n + 3He -> p + T, whose energetic end products give a distinctive electronic pulse in the counter.I've been particularly involved with the 3He-filled tubes, which unfortunately have some tiny amount of radioactive material in the walls: occasional decays of U, Th, and Po nuclei in the tubes lead to pulses which, in some poorly-understood cases, look just like n+3He->p+T pulses. I've been involved in a) testing counters aboveground in unusual ways, and b) using known counter-to-counter symmetries across the whole submerged array as a constraint on the data. SNO also detects throughgoing muons from atmospheric neutrinos. In particular, it is sensitive to muon neutrinos coming from just above the horizon, which (as it so happens) come from neutrinos which have traveled 300-1000 km between their creation (in the stratosphere) and the SNO detector. These muons allow SNO (despite its comparatively small size) to do an interesting neutrino oscillation measurement. I have been involved adding a position- and angle-sensitive array to the SNO cavern, which has detected 36 muons in coincidence with SNO. This data will contribute to the atmospheric analysis. |
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Stable strangelets
Suppose that the strangelet (a hypothetical ball of up, down, and strange quarks---a sort of lambda hyperon/nucleus) is, in fact, energetically stable. (This may or may not be true; an accurate calculation is beyond the abilities of today's nuclear theory, and experiments have been inconclusive.) What are the implications? Most dramatically, neutron stars should actually be giant strongly-bound (as well as gravitationally bound) objects, sometimes called "strange stars". Strange star collisions should eject stable lumps of strange matter of various sizes, possibly including nucleus-scale things with masses of 100-10000 amu---we call these lumps "strangelets". If strangelets are produced anywhere in the galaxy, they should populate the whole galaxy via cosmic rays, and we should find them on the Earth and the Moon.
In addition to searching for strangelets on Earth (see my paper "Cosmic-ray strangelets in the Earth's atmosphere", JHEP 2007.), I am collaborating on a search for strangelets in lunar soil (nucl-ex/0605010) using the WNSL accelerator at Yale. |
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Other projects/interests |
The decay of tritium, T -> 3He + e + neutrino, releases only 18,600 eV of energy. If you start with the tritium nucleus at rest, the final state usually involves the 3He nearly at rest, and the electron and neutrino sharing most of the 18,600 eV. What's the highest possible electron energy? The highest electron energy happens when the electron recoils entirely against the 3He nucleus, leaving the neutrino at rest. (If the electron goes to the left, something has to move right in order to balance out the electron's momentum. If the light neutrino does any of the balancing, it requires a lot of energy, since E = p^2/2m and the neutrino's m is small. If the heavy 3He does the balancing, it requires very little energy.) Of course, this configuration is exactly the same as the (non-existent) decay T->3He+e ... except that the neutrino has a small rest mass, which will be seen to be missing from the e energy.
All available gravitation data (rotation curves, hot gas trapped in clusters, weak lensing, strong lensing, halo star orbits ... ) and cosmology data (structure formation, cosmic microwave background fits) suggests that 75% of the mass of galaxies consists of slow-moving, collisionless particles. Particle physicists have a very convincing idea of what these particles might be: they're the neutral particles predicted by the "supersymmetry" extension to the Standard Model. If we're right about this, we expect to see these particles occasionally bump into ordinary matter nuclei, causing tiny (10-100 keV) energy depositions of a very unusual type. The collisions are so rare, though, that one needs to operate a hundred-kilogram detector for several years---and one may never mistake the dark-matter-type events for the much more common radioactive decays. Several ideas for such detectors are gaining momentum, and may be built in deep-underground labs in the next few years.
There are only a few chapters of Jackson's "Electrodynamics" which come up regularly in discussions of particle detection. We observe particle deflection in magnetic fields; we observe (usually classical or semiclassical) Cerenkov radiation and transition radiation; we observe Bethe-Bloch ionization. What about synchrotron/cyclotron radiation? In principle, it contains information about the particle's mass and velocity. I am interested in exploiting this information---and the advanced techniques used to detect and analyze radio waves in, e.g., atomic physics, astronomy and communications---to do nuclear physics and astrophysics experiments. Avenues of inquiry: