The picture on the left is a snap shot of the SNO event display from less than five minutes ago. The roughly circular cluster of orange dots are SNO light detectors that have "seen" light inside the detector all within 20 nanoseconds of each other. The event is probably caused by a radioactive decay of radioactive salt in SNO.
What the heck is radioactive salt doing in the detector you ask? We put it in a few days ago to help understand our detector's response to other sources of radioactive background and also to understand our response to neutrons. Sodium-24 is the radioactive isotope in the salt (NaCl = sodium chloride).
Fortunately, the half-life of sodium-24 is only 15 hours, so in a couple of weeks, there will be very little radioactivity left. The sodium-24 decays by first emitting an electron (and an anti-neutrino which we don't see). This changes the sodium-24 nucleus into a magnesium-24 nucleus in an "excited state". Basically, the new nucleus is unstable so very quickly emits two gamma rays. One of these gamma rays can sometimes split one of SNO's heavy hydrogen nuclei (the "heavy" part of SNO's heavy water) into a neutron and a proton. This neutron then sticks or "captures" onto another heavy hydrogen nucleus. This in turn produces a high energy gamma ray which is visible in the SNO detector after it scatters an electron. The electron actually emits the light which is seen by the light detectors.
Phew! That's not so straightforward is it. My apologies - I didn't want to lie to you about what really goes on inside SNO! Basically, for these radioactive salt calibration runs, we see the result of neutrons capturing on nuclei in the detector. Seeing lots of these events helps us to understand better how our detector works. Like a piano tuner tunes a piano to get just the right sound, we are tuning SNO to make it run as well as possible.
va = 6*4; //LR(n)
Posted by: m.visaya | November 17, 2005 at 08:31 AM