Rosetta is an ESA Cornerstone mission (launch in 2003) which aims to explore the nucleus and coma of the comet P/Wirtanen by means of an orbiting spacecraft (the Orbiter) and a probe which attaches itself to the surface (the Lander).
One of the experiments selected for the Rosetta Lander (formerly known as RoLand) is "MUPUS". This suite of sensors is designed to measure some properties of the near-surface material, including temperature profile, thermal conductivity, mechanical strength and density, and the evolution of these parameters with time. Interpretation of the results will permit examination of the energy balance across the nucleus surface and constrain models of the formation and evolution of cometary material.
MSSL has been participating in the MUPUS instrument consortium (led by the University of Münster) in the development of a densitometer for the Rosetta lander. Data from the MUPUS densitometer will provide an important ingredient in the analysis of the comet.
Further information and news on MUPUS is available at http://saturn.uni-muenster.de/~seiferl/muphome.html
Initially the proposed density-measuring device used a novel low energy Compton backscatter technique (Ball et al., 1996). In this technique, the surface is irradiated with a collimated photon source. Photons enter the material and may undergo scattering interactions and re-emerge some distance away - these are analysed by one or more detectors. Backscatter density gauges are found numerous terrestrial applications and were also employed for surface density measurements on Mars and Venus (Surkov 1976, 1990). Extension of the technique to low energy is original to this work.
It was originally proposed that the instrument be incorporated into one of the Lander's feet. The source was to be 60 keV 241Am with Silicon detector.
The concept has since evolved to combine the density measurement with the main MUPUS probe. The present design has a 137Cs source at the tip of the probe, the 662 keV radiation from which is viewed by detectors at the surface. The baseline detector type is now Cadmium Telluride (CdTe). The gradual insertion of the MUPUS probe by its hammering mechanism provides an opportunity to measure density at a number of depths.
In this configuration, the linear attenuation of the radiation is measured by counting the photons which are not scattered. The degree of attenuation is almost independent of composition. This is because the dominant interaction at 662 keV is Compton scattering and its cross section is insensitive to composition . (The cross-section for Compton scattering being proportional to electron number density, and the ratio of mass number to atomic number is approximately the same for all elements).
A detector system is required to sense the un-scattered radiation (662 keV) from the source.
The requirements of the detector system are to: permit density measurement in a "reasonable" time, be compact and have low mass, operate in the temperature range at the cometary surface, and withstand the physical environment of the mission, particularly : vibration, radiation, and mission duration. An important additional constraint is the temperature expected at the surface of the nucleus, which may fall as low as 50 K (night side at with the comet at 3 A.U. from the sun) or could rise to 300 K (full sunlight at perihelion).
A photon-counting system is envisaged which consists of: a Detector and housing, Shielding if necessary, Bias voltage unit, Preamplifier, and Pulse processing electronics [shaping, discriminator, counters]. The ability to perform energy-measurement (spectroscopy) is not necessary, but some energy-discrimination is useful for the rejection of scattered radiation.
The main candidate types of detector are: Gaseous, Scintillator and Semiconductor.
Gaseous Detectors (e.g. ionisation counter or proportional counter) have poor efficiency at 662 keV, and relatively high volume and mass. A high voltage unit is needed in addition to the counter itself. These detectors must be rejected due to the combined mass and bulk of the detector and the voltage supply.
Scintillators may also be considered. A crystal of scintillator material is mounted to a detector which senses the light emitted by the crystal when it is struck by a gamma-ray photon. Common scintillators emit light which can be detected by a silicon photodiode. The spectrum and quantity of emitted light is related in a complex way to the incident photon energy, therefore even crude spectroscopy is not straightforward. Otherwise scintillation detectors remain a viable technology.
Semiconductor photon detectors (the majority of which are in a junction diode configuration) are used widely in gamma-ray analysis. The commonest detectors are based on pure Germanium crystals, which for optimum spectroscopic performance must be cyrogenically cooled, and are typically large devices (~200 cm3). Silicon-based detectors operate in a similar way, but since their planar geometry (a few cm2 × a few mm) and lower atomic number mean that they are considerably less efficient for high energy photons. Both these technologies offer significant difficulties for MUPUS. Un-cooled Silicon photodiodes (as per the original baseline) are possible, although their performance is likely to degrade in the upper temperature regime near perihelion.
Compound-material semiconductors offer many attractive features. Some of these materials have combined a high average atomic number (good gamma-ray efficiency) with a high bandgap energy (low leakage current hence no need for cooling). Of these technologies, Cadmium Telluride (CdTe) and Cadmium Zinc Telluride (CZT) seem to be the most promising. These are known as room-temperature detectors since they offer good spectroscopic performance at temperatures in the range 0 °C to 40 °C. Recently these materials have become commercially mature and several manufacturers now offer a wide range of pre-packaged detectors.
Having obtained a sample of a spectrometer grade CdTe detector, we set up a pulse height analysis system using standard nuclear spectroscopy modules. Laboratory work has been focused on two areas: a) elaboration of the original technique of low energy (~60 keV) Compton backscatter, and b) tests of the present configuration - linear attenuation at 662 keV.
A variable energy x-ray source was used to calibrate the detector system and to stimulate the samples. Within this unit, a source of 241Am excites characteristic x-rays from six different targets mounted on a rotary holder. Each target can be presented to the primary source in turn and the characteristic x-rays are emitted through a collimator.
We wished to investigate the backscatter from readily available materials that were: of a wide range of densities ( given in g cm-3); composed of low Z constituents - and of known composition if possible. Water (=1), sugar lumps (=1.26), and expanded polyurethane foam (=0.04) were chosen. Samples were arranged in suitable containers, with the upper surface exposed and horizontal. The source was positioned with the beam pointing vertically into the sample, and the detector was placed in the same plane in contact with the sample surface with lead sheet shielding the detector from direct view of the source.
Pulse height spectra were accumulated with varying distances (the "sonde length") between the source and detector. The spectra were seen to have the following components: direct radiation (through the shielding) from either the 241Am excitation source or from the fluoresced target, counts due to cosmic background, spurious low-energy counts due to electronic noise, and any backscatter from the sample. Additionally, photons entering the detector can cause fluorescence of the detector material which give rise to escape peaks in the spectrum.
Backscatter spectra for Foam and Sugar samples are shown in figure 1. For these data the highest available source energy (Tb target: K =44.23 keV K=50.65 keV) was used to irradiate the sample.
The spectrum of the direct source shows a clear peak at channels 195 and 220 due to complete absorption of the Tb characteristic x-rays, along with an detector escape peak at near channel 90. Higher energy features are due to pulse pile-up. The spectra of the scattered radiation all show a peak near channel 165 (with an associated escape peak ~55). This peak is identified with singly scattered photons.
In all cases there is a clear decrease in the quantity of the return as the sonde length is increased. The amount of returned radiation is less for foam, the material with the lower density. These features are in accordance with expectations. Additionally, there is a small positive energy shift of the scattered radiation with sonde length. This is because the change of direction between the incident and scattered photons is less for the longer sonde lengths, and the Compton energy shift is consequently less.
The experiments have demonstrated the feasibility of a low energy Compton backscatter densitometer. It is clear however that a larger detector would improve the throughput of such an experiment by reducing the number of counts which are lost to the detector escape peaks.
These tests were carried out with a 137Cs source together with the same detector and pulse height analysis system as was used previously.
The detector was then illuminated through samples of water. Spectra were acquired with varying depths of water. The source-detector distance was kept constant. Figure 2 shows a collection of such spectra.
The 662 keV photo-peak (where all the photon's energy is converted to charge) is visible at channel 160. The tail at lower energies is due to incomplete collection of charge in the detector. This is pronounced in this case since the mean absorption depth for these photons is many times the active depth of this detector (the depletion region of the diode).
The shoulder at channels 110-120 (energies less than the labelled "hard region") is characteristic of gamma ray detection at these higher energies where Compton scattering now dominates over the photoelectric effect. The position of the shoulder represents the energy imparted to an electron in the detector material (and hence detected by the system) of a photon which is completely backscattered toward its direction of origin. The continuum extending toward channel 0 is due to scattering events at smaller angles each of which imparts less energy to the scattering electron. An escape peak is visible at channel 19.
Figure 2. Collection of spectra of 662 keV radiation through varying depths of water. Each curve shows the spectrum at the depth indicated by its colour (the data for 0 cm of water is the spectrum of the source through air). The spectra have been scaled to represent equal exposure times.
We have demonstrated the capability to perform density measurements of low density surfaces, either by the Compton backscatter technique or by using an inserted source. Larger detectors will be required to improve the sensitivity and data collection times.
Adaptation to the Rosetta mission requires engineering development of a low power, low mass pulse counting system. The performance of CdTe detectors in the temperature range 100 K - 250 K must be determined experimentally since there is no published information on their performance in this area.
Thanks are due to our partners in this work at the University of Kent: Andrew Ball and John Zarnecki, and to the other members of the MUPUS consortium.
Ball, A. J., Solomon, C. J. and Zarnecki, J. C., A Compton Backscatter Densitometer for the RoLand Comet Lander- design concept and Monte Carlo simulations, Planet. Space Sci. 44(3), 283-293, 1996.
Ball, A. J., Trow, M. W., Smith, A. and Zarnecki, J. C., Laboratory Development of the MUPUS Densitometer for the Rosetta Comet Lander. Poster at the European Geophysical Society, Vienna, 21-25 April 1997. [Abstract available]
Spohn, T., Ball, A., Banaskiewicz, M., Benkhoff, J., Hlond, M., Ip, W.-H., Jaupart, C., Kargl, G., Knollenberg, J., Kömle, N., Kossacki, K. J., Kührt, E., Leese, M. R., Lell, P., Leliwa-Kopystynski, J., Morgan, T., McDonnell, J. A. M., Rott, M., Seiferlin, K., Smith, A., Trow, M., Wright, M., Zarnecki, J. C., Zarnowiecki, T., MUPUS - a suite of small instruments for the RoLand comet lander to study thermal and mechanical properties. Presented at the European Geophysical Society, The Hague, 6-10 May 1996. Abstract in Annal. Geophys. 14 Suppl. III, C818, 1996.
Spohn, T., Ball, A., Banaskiewicz, M., Benkhoff, J., Hlond, M., Grygorczuk, J., Ip, W.-H., Jaupart, C., Kargl, G., Knollenberg, J., Kömle, N., Kossacki, K. J., Kührt, E., Leese, M. R., Lell, P., Leliwa-Kopystynski, J., Morgan, T., McDonnell, J. A. M., Rott, M., Seiferlin, K., Smith, A., Trow, M., Wright, M., Zarnecki, J. C., Zarnowiecki, T., Studies of Thermal and Mechanical Properties with MUPUS - a Suite of Small Instruments for the RoLand Comet Lander. Presented at Asteroids, Comets, Meteors, Versailles, 8-12 July 1996. Abstract in ACM '96 Abstracts Volume, p. 99.
Spohn, T., Ball, A., Banaskiewicz, M., Benkhoff, J., Hlond, M., Ip, W.-H., Jaupart, C., Kargl, G., Knollenberg, J., Kömle, N., Kossacki, K. J., Kührt, E., Leese, M.R., Lell, P., Leliwa-Kopystynski, J., Morgan, T., McDonnell, J. A. M., Rott, M., Seiferlin, K., Smith, A., Trow, M., Wright, M., Zarnecki, J. C., Zarnowiecki, T., MUPUS - a suite of small instruments for the RoLand Comet Lander to study thermal and mechanical properties. Presented at COSPAR 1996, Birmingham, 14-21 July 1996. Abstract in COSPAR 1996 Abstracts Volume, p. 68.
Surkov, Yu. A., Kirnozov, F. F., Khristianov, V. K., Korchuganov, B. N., Glazov, V. N. and Ivanov, V. F., Density of Surface Rock on Venus from Data Obtained by the Venera 10 Automatic Interplanetary Station. Cosmic Res. 14(5), 612-618, 1976.
Surkov, Yu. A., Exploration of terrestrial planets from spacecraft: instrumentation, investigation, interpretation. Ellis Horwood, 1990.