An instrument for in situ comet nucleus surface density profile measurement by gamma ray attenuation

The MUPUS experiment on the Rosetta Lander will measure thermal and mechanical properties as well as the bulk density of the cometary material at and just below the surface of the nucleus of comet 46P/Wirtanen. A profile of bulk density vs. depth will be obtained by measuring the attenuation of 662 keV gamma rays emitted by a 137Cs source. Compton scattering is the dominant interaction process at this energy, the attenuation depending directly on the total number of electrons along the source–detector path. This in turn is approximately proportional to the column density. We report here on the design of the bulk density instrument and the results of related Monte Carlo simulations, laboratory tests and calculations of the instrument's performance. The 137Cs radioisotope source is mounted in the tip of the MUPUS thermal probe—a 10 mm diameter rod, to be hammered into the surface of the nucleus to a depth of ∼370 mm . Two cadmium zinc telluride (CZT) detectors mounted at the top of the probe will monitor the count rate of 662 keV photons. Due to the statistics of photon counting, the integration time required to measure column density to a particular accuracy varies with depth as well as with bulk density. The required integration time is minimised for a material thickness equal to twice the exponential attenuation length. At shallower depths the required time rises due to the smaller fractional change in count rate with varying depth, while at greater depths the reduced count rate demands longer integration times. The former effect and the fact that the first 45 mm of the source–detector path passes not through the comet but through the material of the probe, mean that the first density measurement cannot be made until the source has reached a depth of perhaps 100 mm . The laboratory experiments indicate that at this depth an integration time no less than 348 s (falling to 93.9 s at full penetration) would be required to measure a bulk density of 1000 kg m −3 to 5% accuracy, assuming a source activity of 1.48 mCi (decayed from an initial 2 mCi ). Although solutions involving feedback of the measured bulk density into a time-budgeting algorithm are conceivable, a simple approach where equal time is spent per unit depth may be best, providing an accuracy in bulk density of around 5–20%, for 25 mm slices and the expected range of parameters.