Step away from this thread for a week and abandon all hope of catching up.
I don't know if anyone's already posted this (most likely yes), but I went to the
CRaTER web page and read up on the instrument's description:
CRaTER consists of six silicon detectors in thin/thick pairs separated by sections of Tissue Equivalent Plastic (TEP). The Tissue Equivalent Plastic (such as A-150 manufactured by Standard Imaging) simulates soft body tissue (muscle) and has been used for both ground-based as well as space-based (i.e. Space Station) experiments.
The thin detectors (140 μm) are optimized for high energy deposits and the thick detectors (1000 μm) are optimized for low energy deposits, in particular, for protons. In nominal operating mode, an event is triggered when the energy deposit in any single detector rises above its threshold energy. A measurement is then made of the energy deposit in all six detectors. Directional information can be inferred for events that deposit energy into more than one detector (detection coincidences). Endcaps shield the detectors from protons with less than ~13MeV. Extra mass placed around the edges of the detectors provides additional shielding from some particles which may be able to penetrate through the sides of the instrument.
Go to the web page for a proper diagram of the instrument, but here's some quick-n-dirty ASCII art:
============ --- Deep Space (Zenith) Shield
|||||||||||| --- D1 (148 μm)
++++++++++++ --- D2 (1 mm)
############ --- A150 Tissue Equivalent Plastic (54 mm)
|||||||||||| --- D3 (149 μm)
++++++++++++ --- D4 (1 mm)
############ --- A150 Tissue Equivalent Plastic (27 mm)
|||||||||||| --- D5 (149 μm)
++++++++++++ --- D6 (1 mm)
============ --- Lunar Surface (Nadir) Shield
So, several things that are obvious right off the bat:
- This is why you have combined readings for D1 & D2, D3 & D4, D5 & D6. Each pair is at the same depth of "tissue", each sensor of the pair is optimized for different energies.
- This is why you don't sum up readings from all 6 detectors, because each pair is measuring energies at different "tissue" depths. D1 & D2 give the "skin" dose, D5 & D6 give the "deep tissue" dose.
- As per the description, an event is recorded when the energy deposited in a single detector rises above its threshold energy. Raw events are measured in keV/μm. Obviously, during analysis, these measurements are fed into a mathematical model that spits results out in cGy/day.
One particularly amusing aspect of Tim's confusion (=coughtrollerycough=) is the idea that, because the measurements are reported in cGy/day, that you must sum all the measurements taken on the same day to get the proper cGy/day amount.
cGy/day is a
rate, not a total. It's the
rate of energy absorption
at the time the measurement was taken, and that rate can change from one measurement to the next. It's like saying that if I measure my speed while driving once every couple of minutes and I get 30 mph, 50 mph, 25 mph, and 60 mph, then I really must be going 165 mph.