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Background radiation effects and hazards in planetary instrumentation
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 564 (2006) 559–566 www.elsevier.com/locate/nima
Background radiation effects and hazards in planetary instrumentation Gillian Butchera,, Mark R. Simsa, George Frasera, Go¨star Klingelho¨ferb, Bodo Bernhardtb, Andrew Davidsonc a
Space Research Centre, Michael Atiyah Building, Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK b Institut fu¨r Anorganische und Analytische Chemie, Johannes-Gutenberg-Universita¨t, Staudinger Weg 9, 55128 Mainz, Germany c EADS Astrium, Gunnels Wood Road, Stevenage SG1 2AS, UK Received 17 March 2006; received in revised form 30 March 2006; accepted 31 March 2006 Available online 11 May 2006
Abstract Recent and proposed future planetary missions are becoming increasingly concerned with detailed geochemical assessment, often in a bid to ascertain the presence of water and life supporting geochemical systems. The instruments involved may use some kind of radioactive source, e.g. X-ray fluorescence spectrometry, Mo¨ssbauer spectrometry, neutron scattering. Having radioactive sources on a lander/rover poses various potential problems, in regard to both safety to personnel involved in the building of the instrument and to radiation effects on spacecraft structure and on other instruments. Indeed background radiation effects from one instrument may dominate measurements in another resulting in loss of scientific performance. Drawing on experience with the Beagle 2 probe which contained two instruments with radioactive sources, we present a discussion on the management of radiation hazards and background effects posed by radioactive sources for such planetary missions. r 2006 Elsevier B.V. All rights reserved. PACS: 07.87.tv; 07.89.+b; 87.50.a Keywords: Planetary missions; X-ray radiation; Radiation safety; Radiation shielding; Radiation background
1. Introduction Planetary science missions have progressed from reconnaissance to more detailed explorations, particularly on Mars, with landers and rovers carrying out detailed geochemical assessments [1–4]. The geology and geochemistry can tell us much about the past history of the planet and provide some answers to the key question of whether conditions are or were ever capable of supporting life. A variety of techniques can be used to analyse rock and soil composition: Mo¨ssbauer spectrometry [5,6], X-ray fluorescence spectrometry [7,8], alpha excitation [9,10] and neutron activation [11]. In each case radiation excites the atoms or molecules in the sample and the resulting emitted radiation, which is a signature of the sample, is recorded. Corresponding author. Tel.: +44 116 252 3852; fax: +44 116 252 2464.
E-mail address: [email protected] (G. Butcher). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.03.046
Using a radionuclide stimulus has the advantages of small size and weight, reliability and zero power requirements, all of which are desirable for space missions. The main disadvantage is that the sources will continue to emit radiation at all times. This places additional stress on radiation issues, from hazards to components, instruments and spacecraft to personnel safety. Another factor is the effect of ‘‘unwanted’’ or background radiation on an instrument’s scientific performance. This is applicable not only to radiation emanating from the scientific instruments but also from components such as Radio-isotope Thermal Generators (RTG), which are used for spacecraft power [12]. The additional background radiation, which will be generated from an RTG is considerable and will further increase the hazards and potentially compromise the ability of the payload to conduct its planned observations. In this paper we draw on experience gained on the Beagle 2 probe, which contained two instruments reliant on
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radioactive sources, the Mo¨ssbauer Spectrometer and the X-ray Spectrometer, to discuss the issues present for all such instruments. 2. Radioactivity 2.1. General principles The main aspects of radioactivity that need to be considered are
the energies that are emitted, the source strength or activity, including decay, distance and time.
It is important to be aware of all the energies that are emitted, even those which represent an extremely small percentage of the output (see Section 2.2). A knowledge of the full spectrum of all sources is essential for making accurate calculations and understanding measurements. For any particular type of radiation, such as X-rays, low energy (low penetrating) emissions will be absorbed more easily, which may mean that all the energy is absorbed in the first object in its path, which may affect the material or component’s integrity (see Section 6). In the low energy/ penetrating case shielding is therefore much easier. Higher energies of a given type of radiation are not absorbed so readily, which makes shielding more difficult. On the other hand, they may pass straight through the material without any attenuation or significant damage in which case the radiation field may extend some distance. Where radiation is absorbed by a material it may itself then emit fluorescent X-rays at lower energies. It is the task of the instrument designer to choose a source strength that is as low as possible while maintaining the functionality of the instrument, taking into account the decay of the activity from purchase of the sources to operation on the planet and (planned) end of mission. In operation there is a trade-off between activity and the data collection time. Lower activities mean lower count rates, which require longer integration times to maintain the counting statistics. Radiation falls off as one over the distance squared so the closer an object or person is to the source the more radiation is intercepted. Maintaining as large a distance as practically possible between the source and object is therefore required. The effects of radiation are cumulative so it is essential to restrict the time that the object or person is subjected to the radiation to a minimum. This may mean that at a very early stage in the design of the instrument consideration is given to the accessibility of the sources so that they may be integrated at the last possible moment with ease. Shielding is more effective the closer it is to the source, enabling it to be smaller and therefore lighter. One approach is to consider graded shielding (see for example
Ref. [13]) which consists of a series of layers, the first being a high Z material. The fluorescent X-rays that are emitted by this layer can then be absorbed by a second layer of lower Z material. This layer may also re-emit X-ray fluorescence and another layer may be added and so on until all the radiation is absorbed. The issue and effects of shielding will be discussed in the following sections in the context of the Beagle 2 mission. 2.2. Beagle 2: instrumentation and radioactive sources Beagle 2 was launched in June 2003 on the Mars Express spacecraft and entered the atmosphere of Mars on 25th December 2003, but failed to respond from the Martian surface and was declared lost. The goal of the mission was to establish whether conditions for life were ever present at the landing site, by searching for organic material on and below the surface as well as studying the local inorganic chemistry and mineralogy [8]. Most of the instruments were located on the Position Adjustable Workbench (PAW), which can be seen in Fig. 1. The Mo¨ssbauer Spectrometer (MS) and X-ray fluorescence Spectrometer (XRS) were to provide information on the mineral content and the elemental composition, respectively, of the rocks and soils [14]. Both of these instruments contained radioactive sources to provide the necessary stimulus to the samples and radiation detectors to record the samples’ signature. The MS contained a 57Co source, the details of which are given in Table 1. As can be seen from the table there is a small proportion of the radiation that is emitted at 692 keV, which is not mentioned in some data books. However, as this energy is very penetrating it was virtually impossible to eliminate it so that as shielding was added to reduce the total radiation, and in particular the major lower energy components, this higher energy emission assumed more significance. The most effective shielding (for the energies below 692 keV) was the addition of tantalum plates fitted within the MS cover, positioned around the source. It was necessary, for reasons of safety of
Fig. 1. Beagle 2 lander showing position and close proximity of instruments on PAW.
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Table 1 Radioactive sources on Beagle 2 Energies emitted (keV) Mo¨ssbauer 57 Co 122 136 14.4 692 XRS 55 Fe 109
Cd
5.9 6.4 22.2 24.9 88
Relative intensities
Half-life
Quantity
Activity at integration
Activity arriving at Mars
0.85 0.11 0.085 0.0015
271 days
1
340 MBq
130 MBq
2.74 years
2
1.27 years
2
77 MBq per source 9 MBq per source
60 MBq per source 5 MBq per source
0.05
Fig. 2. 3D CAD model showing the Mo¨ssbauer Spectrometer with shutter closed.
personnel and components, to have a shutter over the front of the MS as shown in Fig. 2, consisting of 3 mm thick Ta. The shutter was operated by a worm drive mechanism able to resist a force of 53 N (exceeding expected mission load and shock levels) so that the shutter could not be accidentally moved. The inside of the shutter could also be used as a calibration target for the MS. The XRS contained two types of sources, 55Fe and 109Cd with two of each type, the details of which are given in Table 1. The low energies and low activities of the XRS sources meant that compared with the MS the XRS was not so much of a hazard. 3. Consideration of the radiation hazards and effects There are three aspects of the hazards associated with radiation that need to be considered:
the safety of personnel, the integrity of components and the effect on scientific performance.
The first aspect is the safety of personnel working on the instrument and the spacecraft after the sources have been integrated. The radiation from all sources needs to be assessed for all the different stages of the spacecraft build. The flow diagram of Fig. 3 shows the various stages of
Fig. 3. Flow of assembly and integration of the Beagle 2 probe.
assembly for Beagle 2, the radiation dose calculations for each stage being given in Section 5. The second aspect is the integrity of components and materials of the spacecraft, which are in close proximity to the sources and could accumulate significant amounts of radiation during flight and then during operations. The effect of radiation on space components [15] usually only considers the radiation from the space environment. However, the radiation from the radioactive sources also needs to be taken into consideration. The main causes of concern are the electronic components where radiation can cause single event upsets or failure of a component. Optical materials may also be affected by radiation, which alters their transmission properties. Also of concern are some of the (carbon-based) materials, such as the spacecraft composite structure and parachute material where the radiation can alter the C–C and C–H bonds resulting in the formation of new compounds and possible reduction of strength through bond breaking. The radiation dose at each of these components needs to be calculated and then classified as a hazard if necessary.
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Third is the contribution of background radiation on an instrument’s performance. For Beagle 2 it was considered that having such a large radiation source as the MS close to the XRS could cause either direct or scattered radiation in the XRS detector or could fluoresce materials in its vicinity. This would all appear as unwanted background to the actual XRS spectra. The design of the PAW took into account the requirement that the MS and XRS were as far apart as possible, given other design considerations. 4. Relevant radiation calculations In order to make a practical assessment of radiation hazards it is necessary to quantify the radiation absorbed by the object under consideration. The parameter of importance for inanimate objects is absorbed dose while for living objects, i.e. humans, it is biological absorbed dose. In practice absorbed dose is given in units of rad (non-SI) while safety requirements are given in Sv/h (SI). For the MS, measurements were taken of the radiation from the shuttered instrument, the resulting polar flux diagram being shown in Fig. 4. As these measurements were taken at a distance of 20 cm the calculation for other distances has to be referenced to this value. To calculate the absorbed dose from the MS the following equation was then used: 2 Y 20 D ¼ Ac T expðmi ri ti Þ (1) d i rad / h 1.5x10-4 120
60
1x10-4 150
where D is the total absorbed dose in rad, A the absorbed dose rate in rad/h, from Fig. 4 for the appropriate direction, c the relative absorption correction factor, d the distance between source and absorbing material in cm, T the time over which the radiation is accumulated in hours, m the mass attenuation co-efficient in cm2/g, at the energy of the incident radiation, r the mass density coefficient in g/cm3, t the thickness of the material in cm, subscript i refers to a particular layer of material. The starting point for the XRS calculations was the intensity of the sources, from the manufacturer’s catalogue [16] and supplied certificates. To calculate the absorbed dose from the XRS the following equation was used, for 55 Fe and 109Cd separately the results then summed together: I T 3600 ½1 expðmÞ (2) 6:25 107 d 2 where D is the absorbed dose in rad, I the intensity of the source in MeV/s/sr from Ref. [16], T the time over which the radiation is accumulated in hours, d the distance between source and absorbing material in cm, m the mass attenuation co-efficient in cm2/g, at the energy of the incident radiation. The biological absorbed dose or dose rate can be calculated from the equation D¼
H ¼ DQ
(3)
where H is the biological absorbed dose rate in rem/h (Sv/ h), D the absorbed dose rate in rad/h (Gy/h), Q the quality factor, which for X- and g-rays ¼ 1. In all cases where calculations are being performed, while it is important to model the conditions as accurately as possible, it is also important to take the worst-case scenario. For example, the source strength should be taken as the maximum quoted with largest uncertainty and in some cases it may be more appropriate to ignore radioactivity decay effects, given the decay constants involved.
30
5. Personnel radiation safety 5x10-5
5.1. Radiation safety during source integration 180
0
0
210
330
240
300 270
Fig. 4. Measured flux diagram of radiation at all energies at a distance of 20 cm from the shuttered Mo¨ssbauer instrument (for a source strength of 0.34 GBq). The front of the instrument points toward 01.
For the radiation safety of personnel, measurements should be taken throughout the development, build and integration programme of the radiation levels around the instrument(s) containing the radioactive sources. One should use Geiger counters or other radiation monitors that are sufficiently sensitive to all the radiation energies of interest and that are calibrated regularly and accurately. For Beagle 2, during 57Co source integration into the MS, due to the design of the instrument it was not practically possible to have any temporary (for integration) shielding around the source and consequently there was no shielding from when the source was removed from its safe storage container to its insertion into the MS. The source integration was carried out by trained radiation personnel
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Fig. 5. XRS showing source location in CFRP cap. 109Cd sources located at 901 to 55Fe, inserted from rear of CFRP cap.
in a controlled access area using a special tool and the process was practised many times with a dummy source capsule to minimise time spent carrying the operation out. Once inserted into the MS a shutter was positioned over the front of the instrument and blocked most of the direct radiation. As can be seen from the measured flux diagram of Fig. 4 there is still radiation present, with a maximum value of 1.5 104 rad/h at 60–801 to the forward direction. It was calculated that the amount of biological dose received by the operator during integration of the sources of 0.2 mSv was a very small fraction of the allowed yearly limit, 2 mSv/year (see Appendix A). The XRS was designed such that the sources were located in a removable carbon fibre reinforced plastic (CFRP) cap, as shown in Fig. 5. The sources were inserted into the CFRP cap in a controlled access area by radiationtrained personnel, using tweezers to maximise distance and minimising time spent carrying out the operation. A lead cover shield was fixed over the front of the CFRP cap, which was designed to absorb all the radiation. This shielded unit was red-tagged, which meant that when it was integrated into the PAW the cover shield was removed at the last possible moment (within the restrictions imposed by other last minute integration and test activities). Calculations were confirmed by measurement with a broadband Geiger counter that there was negligible radiation emitted from the shielded XRS. Once inside the lander and with the shield removed it was calculated that there was no radiation hazard to personnel from the XRS, the low energy lines all being absorbed by the lander structure (see Section 6), which again was confirmed by measurement. 5.2. Radiation safety during instrument integration The process of integration of the instrument(s) and subsequent Assembly, Integration and Test (AIT) activities
563
up to closeout of a planetary lander is potentially the most hazardous as there is minimal shielding afforded by any other structures or materials and non-radiation personnel may be working on the probe at this stage. For Beagle 2 the XRS unit, once installed into its position in the lander, emitted no radiation and so did not need to be considered in these calculations. The exposure for a person carrying out the integration of the MS onto the PAW was calculated using the equations in Section 4. The absorbed dose rate was calculated from Fig. 4, taking the maximum value of radiative flux of 1.5 104 rad/h. For the absorbing material, which is biological, the material is taken as water which means that the relative absorption factor is 1. The total length of time the operation would take was estimated as 8 h and the distance of the person from the MS of 10 cm was assumed. It was assumed that there was no shielding from any other components or structure. Putting these values, which are tabulated in Table 2, into Eq. (1) gives the total absorbed dose as 4.8 mrad which converts, using Eq. (3), to a biological absorbed dose for X-rays of 4.8 mrem. With a conversion of 1 rem equal to 10 mSv the exposure was estimated to be 48 mSv, which is a small fraction of the allowed yearly dose of 2 mSv/year (see Appendix A). At the stage of integration of the PAW into the lander there was some shielding of the radiation from the MS from the PAW structure and PLUTO instrument (see Fig. 1). For worst-case hazard calculations it was assumed that there was no further shielding. From integration to closing of the lander non-radiation personnel had various AIT activities to carry out on the open lander, for whom the biological dose and exposure was calculated. Again the worst-case value of radiated flux was used, a fixed distance of 10 cm was assumed and an estimation made of the amount of time that personnel would carry out these activities (7 h a day, over 10 days). The exposure was calculated to be 420 mSv, as given in Table 2. In the unlikely event that one person worked so closely to the MS at the ‘‘worst’’ angle and for the full period, their absorbed dose was still within the UK legal limits for the year (see Appendix A). 5.3. Radiation safety during lander integration The Beagle 2 lander was encased in its parachute and airbag material and integrated into the probe. This process involved non-radiation personnel working closely round the lander for long periods of time. The worst case value of radiated flux was used, which assumed no shielding from the lander structure, personnel were at a fixed distance of 20 cm and an estimation made of the amount of time that personnel would carry out these activities. The exposure was calculated to be 147 mSv, as given in Table 2, within the UK legal requirements (see Appendix A). The exposure for further activities can similarly be calculated, right up until launch. Table 2 shows the results
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Table 2 Beagle 2 personnel radiation exposure based on continuous exposure at a fixed body distance from MS, for different stages of the build Team
UL Astrium
TLS Astrium
Starsem
Task
Hours per day
Days
Total exposure Based on body Estimated time (h) distance of source dose (cm) (mSv/h)
Biological absorbed dose
Per task mSv
mSv 0.048 0.42 0.147 0.02352 0.02352
PAW integration Lander closeout Probe closeout TLS integration Launch site integration Arming
8 7 7 7 7
1 10 14 14 14
8 70 98 98 98
10 10 20 50 50
6 6 1.5 0.24 0.24
48 420 147 23.52 23.52
7
7
49
100
0.06
2.94 0.00294 Total exposure 0.66498
Spacecraft work Launch site integration Launch site work
7 7
14 20
98 140
50 50
0.24 0.24
7
60
420
100
0.06
Launch site work Guard
7 12
84 84
588 1008
100 100
0.06 0.06
23.52 33.6
0.02352 0.0336
25.2 0.0252 Total exposure 0.08232 35.28 60.48
0.03528 0.06048
of the radiation exposure calculations for the various activities for Beagle 2. 5.4. Radiation safety during probe transport At some stage in the build it will be necessary to transport the probe to be attached onto the carrier spacecraft or integrated onto its launcher. The safety requirements for transport require that the absorbed dose is sufficiently low to allow non-radiation personnel to work safely with the package. The IAEA Regulations For The Safe Transport Of Radioactive Material 1996 [18] require the dose rate to be less than 100 mSv/h at 10 cm from the outer surface of the unpackaged item, i.e. at 10 cm from the surface of the probe. The levels for Beagle 2 were calculated for several points round the probe, as shown in Fig. 6, none of which exceeded 1 mSv/h at 10 cm, well within the limits required. Beagle 2 was transported within its own dedicated transport container within a standard ISO container. 6. Structure and component integrity Spacecraft electronics are susceptible to radiation damage and hence radiation hardness testing is one of the many environmental tests that space instruments and components may have to undergo [15] depending on the expected mission environment. It is also necessary to consider the spacecraft structure tolerance to radiation. The external radiation levels, from solar and cosmic protons and neutrons, were calculated for Mars Express to amount to 6 krad [19] over the mission lifetime and it was required that components be radiation hard to 12 krad.
Fig. 6. Schematic of the Beagle 2 probe showing the position and direction of the Mo¨ssbauer instrument and values of radiative flux in mSv/h at 10 cm from the surface of the probe.
The proximity of many of the PAW instruments to the instrument radiation sources meant that the internal radiation had to be added to the external radiation levels to work out if the components were sufficiently radiation hard to withstand the total expected radiation level. Calculations, using the equations in Section 4, were made for the electronic components on the PAW and for the optical filters on the camera, as well as for the common electronics boards. The calculations all showed that the internal radiation levels were sufficiently small (of order 15 rad at the nearest components) that they added very little to the external radiation levels. The other area of potential concern was the tolerance of the spacecraft structure to the radiation levels. Calculations showed that the various layers of the lander structure absorbed about 0.7 krad from the MS source while the
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airbags absorbed 30 rad, both at the 692 keV energy, the levels from the 122 keV emission being much smaller. As there was no shielding in front of the XRS sources and no other components in its path, all the radiation was absorbed in the lander structure: 5 krad was absorbed in the first 75 mm carbon fibre layer (mainly from the 55Fe radiation) and 6 krad was absorbed in the next 1 cm layer of aluminium honeycomb (mainly from the 109Cd radiation). None of these radiation doses, given the materials and the absorption depths in the materials, were sufficiently high to give concern.
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Table 3 Reduction of sensitivity of XRS due to background radiation from MS 57 Co source Element
Detection limit No
Si K Zn Rb Zr Y Nb
57
4.05 0.06 85 31 38 38 35
Co
Unit With 30.70 0.25 1381 336 257 322 302
57
Co % % ppm ppm ppm ppm ppm
7. Effects on scientific performance As discussed above radiation may damage electronic components, however, it may also manifest itself as degraded performance of an instrument, for example, CCD detectors will exhibit poorer spectral resolution with dose [17]. Also where there is another instrument that operates by detecting radiation this ‘‘external’’ radiation may appear as unwanted background. In the early stage of development of Beagle 2, measurements were taken of XRS spectra with a 57Co source in close proximity. The results showed that there was a large increase in the background of the XRS spectrum which would reduce its sensitivity, as shown in Fig. 7. It was clear that many of the spectral peaks were swamped by the scatter background, destroying the capability of the XRS to measure some of the elements and reducing its sensitivity to other elements. Table 3 shows the calculated detection limits for various elements in a granite reference sample, showing the reduced sensitivity from when there is no 57Co present to when the source is in place. It can be seen that the sensitivity drops by a factor from 4 to 8 for the major elements and by about 10 for the trace elements. The Compton scatter from the MS is clearly seen in Fig. 7 and its scatter angle can be calculated from the Compton edge (calculated to be 79.51). From measurements with and without the XRS sources in place, it was discovered that most of the added
8. Summary It is clear from the experience of Beagle 2 that when dealing with radioactive sources in planetary instrumentation hazards and effects and possible solutions should be considered early on in the design phase of the spacecraft, taking into account all energies from all sources. Particular issues to consider include:
10000 Ag K
Ec
Au K
100
Compton Edge
with Co-57
Co-57 122 keV photopeak
10
no Co-57
1 0
20
personnel safety at all stages of the programme, radiation damage to materials and components, including external radiation dose, effects on performance of scientific instruments.
The following steps can be taken to reduce or eliminate these hazards:
1000
counts
background arose from the 57Co 122 keV radiation being scattered off the XRS source capsules, these being high Z and high scatter materials in close proximity to the XRS detector. An estimation of the angle from drawings of the 57 Co source to the XRS sources to the XRS detector was made and the scatter angle was found to be 801751, which corresponded well with the angle calculated using the Compton edge energy above. With the addition of Ta shielding round the MS source, within the instrument body, the leakage radiation was reduced considerably so that there was no noticeable background in the XRS spectra from the MS.
40
60
80
100
120
140
energy / keV
Fig. 7. XRS spectrum of a granite sample showing enhanced background from MS Co-57.
keep source strengths as low as possible that still allow the science to be performed, minimise time spent in proximity to the sources, carry out dry runs to practise manoeuvres to minimise time, design ease of access of source insertion and instrument integration, maximise distance to the sources, insert shielding close to sources, use graded shielding where necessary.
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In practice it is also advisable to
make worst case calculations,
but more importantly to
take measurements with a properly calibrated instrument.
Acknowledgements We would like to acknowledge the support of PPARC for funding the work at the University of Leicester. Discussions with John Scott, Radiation Safety Officer of the University of Leicester were particularly helpful. Thanks also to the drawing office team at Astrium for providing detailed drawings of the lander and probe. Appendix A. Relevant radiation regulations The regulations relevant to Beagle 2 and the UK, concerning the radiation dose are: The UK Ionising Radiation Regulations 1999, Schedule 1, Regulations 6(1) and 13(3) state that ‘‘work with ionising radiation shall not be required to be notified in accordance with regulation 6 when y the apparatus does not under normal operating conditions cause a dose rate of more than 1 mSv/h at a distance of 0.1 m from any accessible surface’’ [20]. The UK Ionising Radiation Regulations 1999, Schedule 4, Regulation 11 state that ‘‘the limit on effective dose for any person other than an employee or trainee, including any person below the age of 16, shall be 1 mSv in any calendar year’’ [20]. A controlled access area is designated when the dose rate is more than 7 mSv/h at a distance of 1 m. UK Regulations for the Safe Transport of Radioactive Material 1996 state that for an excepted package the radiation level at 10 cm from the external surface of any unpackaged instrument shall not exceed 0.1 mSv/h [21].
The IAEA Regulations For The Safe Transport Of Radioactive Material 1996 require that for an excepted package the biological absorbed dose must be o100 mSv/h at 10 cm from the external surface of the unpackaged instrument [18].
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Nuclear Instruments and Methods in Physics Research A 564 (2006) 559–566 www.elsevier.com/locate/nima
Background radiation effects and hazards in planetary instrumentation Gillian Butchera,, Mark R. Simsa, George Frasera, Go¨star Klingelho¨ferb, Bodo Bernhardtb, Andrew Davidsonc a
Space Research Centre, Michael Atiyah Building, Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK b Institut fu¨r Anorganische und Analytische Chemie, Johannes-Gutenberg-Universita¨t, Staudinger Weg 9, 55128 Mainz, Germany c EADS Astrium, Gunnels Wood Road, Stevenage SG1 2AS, UK Received 17 March 2006; received in revised form 30 March 2006; accepted 31 March 2006 Available online 11 May 2006
Abstract Recent and proposed future planetary missions are becoming increasingly concerned with detailed geochemical assessment, often in a bid to ascertain the presence of water and life supporting geochemical systems. The instruments involved may use some kind of radioactive source, e.g. X-ray fluorescence spectrometry, Mo¨ssbauer spectrometry, neutron scattering. Having radioactive sources on a lander/rover poses various potential problems, in regard to both safety to personnel involved in the building of the instrument and to radiation effects on spacecraft structure and on other instruments. Indeed background radiation effects from one instrument may dominate measurements in another resulting in loss of scientific performance. Drawing on experience with the Beagle 2 probe which contained two instruments with radioactive sources, we present a discussion on the management of radiation hazards and background effects posed by radioactive sources for such planetary missions. r 2006 Elsevier B.V. All rights reserved. PACS: 07.87.tv; 07.89.+b; 87.50.a Keywords: Planetary missions; X-ray radiation; Radiation safety; Radiation shielding; Radiation background
1. Introduction Planetary science missions have progressed from reconnaissance to more detailed explorations, particularly on Mars, with landers and rovers carrying out detailed geochemical assessments [1–4]. The geology and geochemistry can tell us much about the past history of the planet and provide some answers to the key question of whether conditions are or were ever capable of supporting life. A variety of techniques can be used to analyse rock and soil composition: Mo¨ssbauer spectrometry [5,6], X-ray fluorescence spectrometry [7,8], alpha excitation [9,10] and neutron activation [11]. In each case radiation excites the atoms or molecules in the sample and the resulting emitted radiation, which is a signature of the sample, is recorded. Corresponding author. Tel.: +44 116 252 3852; fax: +44 116 252 2464.
E-mail address: [email protected] (G. Butcher). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.03.046
Using a radionuclide stimulus has the advantages of small size and weight, reliability and zero power requirements, all of which are desirable for space missions. The main disadvantage is that the sources will continue to emit radiation at all times. This places additional stress on radiation issues, from hazards to components, instruments and spacecraft to personnel safety. Another factor is the effect of ‘‘unwanted’’ or background radiation on an instrument’s scientific performance. This is applicable not only to radiation emanating from the scientific instruments but also from components such as Radio-isotope Thermal Generators (RTG), which are used for spacecraft power [12]. The additional background radiation, which will be generated from an RTG is considerable and will further increase the hazards and potentially compromise the ability of the payload to conduct its planned observations. In this paper we draw on experience gained on the Beagle 2 probe, which contained two instruments reliant on
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radioactive sources, the Mo¨ssbauer Spectrometer and the X-ray Spectrometer, to discuss the issues present for all such instruments. 2. Radioactivity 2.1. General principles The main aspects of radioactivity that need to be considered are
the energies that are emitted, the source strength or activity, including decay, distance and time.
It is important to be aware of all the energies that are emitted, even those which represent an extremely small percentage of the output (see Section 2.2). A knowledge of the full spectrum of all sources is essential for making accurate calculations and understanding measurements. For any particular type of radiation, such as X-rays, low energy (low penetrating) emissions will be absorbed more easily, which may mean that all the energy is absorbed in the first object in its path, which may affect the material or component’s integrity (see Section 6). In the low energy/ penetrating case shielding is therefore much easier. Higher energies of a given type of radiation are not absorbed so readily, which makes shielding more difficult. On the other hand, they may pass straight through the material without any attenuation or significant damage in which case the radiation field may extend some distance. Where radiation is absorbed by a material it may itself then emit fluorescent X-rays at lower energies. It is the task of the instrument designer to choose a source strength that is as low as possible while maintaining the functionality of the instrument, taking into account the decay of the activity from purchase of the sources to operation on the planet and (planned) end of mission. In operation there is a trade-off between activity and the data collection time. Lower activities mean lower count rates, which require longer integration times to maintain the counting statistics. Radiation falls off as one over the distance squared so the closer an object or person is to the source the more radiation is intercepted. Maintaining as large a distance as practically possible between the source and object is therefore required. The effects of radiation are cumulative so it is essential to restrict the time that the object or person is subjected to the radiation to a minimum. This may mean that at a very early stage in the design of the instrument consideration is given to the accessibility of the sources so that they may be integrated at the last possible moment with ease. Shielding is more effective the closer it is to the source, enabling it to be smaller and therefore lighter. One approach is to consider graded shielding (see for example
Ref. [13]) which consists of a series of layers, the first being a high Z material. The fluorescent X-rays that are emitted by this layer can then be absorbed by a second layer of lower Z material. This layer may also re-emit X-ray fluorescence and another layer may be added and so on until all the radiation is absorbed. The issue and effects of shielding will be discussed in the following sections in the context of the Beagle 2 mission. 2.2. Beagle 2: instrumentation and radioactive sources Beagle 2 was launched in June 2003 on the Mars Express spacecraft and entered the atmosphere of Mars on 25th December 2003, but failed to respond from the Martian surface and was declared lost. The goal of the mission was to establish whether conditions for life were ever present at the landing site, by searching for organic material on and below the surface as well as studying the local inorganic chemistry and mineralogy [8]. Most of the instruments were located on the Position Adjustable Workbench (PAW), which can be seen in Fig. 1. The Mo¨ssbauer Spectrometer (MS) and X-ray fluorescence Spectrometer (XRS) were to provide information on the mineral content and the elemental composition, respectively, of the rocks and soils [14]. Both of these instruments contained radioactive sources to provide the necessary stimulus to the samples and radiation detectors to record the samples’ signature. The MS contained a 57Co source, the details of which are given in Table 1. As can be seen from the table there is a small proportion of the radiation that is emitted at 692 keV, which is not mentioned in some data books. However, as this energy is very penetrating it was virtually impossible to eliminate it so that as shielding was added to reduce the total radiation, and in particular the major lower energy components, this higher energy emission assumed more significance. The most effective shielding (for the energies below 692 keV) was the addition of tantalum plates fitted within the MS cover, positioned around the source. It was necessary, for reasons of safety of
Fig. 1. Beagle 2 lander showing position and close proximity of instruments on PAW.
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Table 1 Radioactive sources on Beagle 2 Energies emitted (keV) Mo¨ssbauer 57 Co 122 136 14.4 692 XRS 55 Fe 109
Cd
5.9 6.4 22.2 24.9 88
Relative intensities
Half-life
Quantity
Activity at integration
Activity arriving at Mars
0.85 0.11 0.085 0.0015
271 days
1
340 MBq
130 MBq
2.74 years
2
1.27 years
2
77 MBq per source 9 MBq per source
60 MBq per source 5 MBq per source
0.05
Fig. 2. 3D CAD model showing the Mo¨ssbauer Spectrometer with shutter closed.
personnel and components, to have a shutter over the front of the MS as shown in Fig. 2, consisting of 3 mm thick Ta. The shutter was operated by a worm drive mechanism able to resist a force of 53 N (exceeding expected mission load and shock levels) so that the shutter could not be accidentally moved. The inside of the shutter could also be used as a calibration target for the MS. The XRS contained two types of sources, 55Fe and 109Cd with two of each type, the details of which are given in Table 1. The low energies and low activities of the XRS sources meant that compared with the MS the XRS was not so much of a hazard. 3. Consideration of the radiation hazards and effects There are three aspects of the hazards associated with radiation that need to be considered:
the safety of personnel, the integrity of components and the effect on scientific performance.
The first aspect is the safety of personnel working on the instrument and the spacecraft after the sources have been integrated. The radiation from all sources needs to be assessed for all the different stages of the spacecraft build. The flow diagram of Fig. 3 shows the various stages of
Fig. 3. Flow of assembly and integration of the Beagle 2 probe.
assembly for Beagle 2, the radiation dose calculations for each stage being given in Section 5. The second aspect is the integrity of components and materials of the spacecraft, which are in close proximity to the sources and could accumulate significant amounts of radiation during flight and then during operations. The effect of radiation on space components [15] usually only considers the radiation from the space environment. However, the radiation from the radioactive sources also needs to be taken into consideration. The main causes of concern are the electronic components where radiation can cause single event upsets or failure of a component. Optical materials may also be affected by radiation, which alters their transmission properties. Also of concern are some of the (carbon-based) materials, such as the spacecraft composite structure and parachute material where the radiation can alter the C–C and C–H bonds resulting in the formation of new compounds and possible reduction of strength through bond breaking. The radiation dose at each of these components needs to be calculated and then classified as a hazard if necessary.
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Third is the contribution of background radiation on an instrument’s performance. For Beagle 2 it was considered that having such a large radiation source as the MS close to the XRS could cause either direct or scattered radiation in the XRS detector or could fluoresce materials in its vicinity. This would all appear as unwanted background to the actual XRS spectra. The design of the PAW took into account the requirement that the MS and XRS were as far apart as possible, given other design considerations. 4. Relevant radiation calculations In order to make a practical assessment of radiation hazards it is necessary to quantify the radiation absorbed by the object under consideration. The parameter of importance for inanimate objects is absorbed dose while for living objects, i.e. humans, it is biological absorbed dose. In practice absorbed dose is given in units of rad (non-SI) while safety requirements are given in Sv/h (SI). For the MS, measurements were taken of the radiation from the shuttered instrument, the resulting polar flux diagram being shown in Fig. 4. As these measurements were taken at a distance of 20 cm the calculation for other distances has to be referenced to this value. To calculate the absorbed dose from the MS the following equation was then used: 2 Y 20 D ¼ Ac T expðmi ri ti Þ (1) d i rad / h 1.5x10-4 120
60
1x10-4 150
where D is the total absorbed dose in rad, A the absorbed dose rate in rad/h, from Fig. 4 for the appropriate direction, c the relative absorption correction factor, d the distance between source and absorbing material in cm, T the time over which the radiation is accumulated in hours, m the mass attenuation co-efficient in cm2/g, at the energy of the incident radiation, r the mass density coefficient in g/cm3, t the thickness of the material in cm, subscript i refers to a particular layer of material. The starting point for the XRS calculations was the intensity of the sources, from the manufacturer’s catalogue [16] and supplied certificates. To calculate the absorbed dose from the XRS the following equation was used, for 55 Fe and 109Cd separately the results then summed together: I T 3600 ½1 expðmÞ (2) 6:25 107 d 2 where D is the absorbed dose in rad, I the intensity of the source in MeV/s/sr from Ref. [16], T the time over which the radiation is accumulated in hours, d the distance between source and absorbing material in cm, m the mass attenuation co-efficient in cm2/g, at the energy of the incident radiation. The biological absorbed dose or dose rate can be calculated from the equation D¼
H ¼ DQ
(3)
where H is the biological absorbed dose rate in rem/h (Sv/ h), D the absorbed dose rate in rad/h (Gy/h), Q the quality factor, which for X- and g-rays ¼ 1. In all cases where calculations are being performed, while it is important to model the conditions as accurately as possible, it is also important to take the worst-case scenario. For example, the source strength should be taken as the maximum quoted with largest uncertainty and in some cases it may be more appropriate to ignore radioactivity decay effects, given the decay constants involved.
30
5. Personnel radiation safety 5x10-5
5.1. Radiation safety during source integration 180
0
0
210
330
240
300 270
Fig. 4. Measured flux diagram of radiation at all energies at a distance of 20 cm from the shuttered Mo¨ssbauer instrument (for a source strength of 0.34 GBq). The front of the instrument points toward 01.
For the radiation safety of personnel, measurements should be taken throughout the development, build and integration programme of the radiation levels around the instrument(s) containing the radioactive sources. One should use Geiger counters or other radiation monitors that are sufficiently sensitive to all the radiation energies of interest and that are calibrated regularly and accurately. For Beagle 2, during 57Co source integration into the MS, due to the design of the instrument it was not practically possible to have any temporary (for integration) shielding around the source and consequently there was no shielding from when the source was removed from its safe storage container to its insertion into the MS. The source integration was carried out by trained radiation personnel
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Fig. 5. XRS showing source location in CFRP cap. 109Cd sources located at 901 to 55Fe, inserted from rear of CFRP cap.
in a controlled access area using a special tool and the process was practised many times with a dummy source capsule to minimise time spent carrying the operation out. Once inserted into the MS a shutter was positioned over the front of the instrument and blocked most of the direct radiation. As can be seen from the measured flux diagram of Fig. 4 there is still radiation present, with a maximum value of 1.5 104 rad/h at 60–801 to the forward direction. It was calculated that the amount of biological dose received by the operator during integration of the sources of 0.2 mSv was a very small fraction of the allowed yearly limit, 2 mSv/year (see Appendix A). The XRS was designed such that the sources were located in a removable carbon fibre reinforced plastic (CFRP) cap, as shown in Fig. 5. The sources were inserted into the CFRP cap in a controlled access area by radiationtrained personnel, using tweezers to maximise distance and minimising time spent carrying out the operation. A lead cover shield was fixed over the front of the CFRP cap, which was designed to absorb all the radiation. This shielded unit was red-tagged, which meant that when it was integrated into the PAW the cover shield was removed at the last possible moment (within the restrictions imposed by other last minute integration and test activities). Calculations were confirmed by measurement with a broadband Geiger counter that there was negligible radiation emitted from the shielded XRS. Once inside the lander and with the shield removed it was calculated that there was no radiation hazard to personnel from the XRS, the low energy lines all being absorbed by the lander structure (see Section 6), which again was confirmed by measurement. 5.2. Radiation safety during instrument integration The process of integration of the instrument(s) and subsequent Assembly, Integration and Test (AIT) activities
563
up to closeout of a planetary lander is potentially the most hazardous as there is minimal shielding afforded by any other structures or materials and non-radiation personnel may be working on the probe at this stage. For Beagle 2 the XRS unit, once installed into its position in the lander, emitted no radiation and so did not need to be considered in these calculations. The exposure for a person carrying out the integration of the MS onto the PAW was calculated using the equations in Section 4. The absorbed dose rate was calculated from Fig. 4, taking the maximum value of radiative flux of 1.5 104 rad/h. For the absorbing material, which is biological, the material is taken as water which means that the relative absorption factor is 1. The total length of time the operation would take was estimated as 8 h and the distance of the person from the MS of 10 cm was assumed. It was assumed that there was no shielding from any other components or structure. Putting these values, which are tabulated in Table 2, into Eq. (1) gives the total absorbed dose as 4.8 mrad which converts, using Eq. (3), to a biological absorbed dose for X-rays of 4.8 mrem. With a conversion of 1 rem equal to 10 mSv the exposure was estimated to be 48 mSv, which is a small fraction of the allowed yearly dose of 2 mSv/year (see Appendix A). At the stage of integration of the PAW into the lander there was some shielding of the radiation from the MS from the PAW structure and PLUTO instrument (see Fig. 1). For worst-case hazard calculations it was assumed that there was no further shielding. From integration to closing of the lander non-radiation personnel had various AIT activities to carry out on the open lander, for whom the biological dose and exposure was calculated. Again the worst-case value of radiated flux was used, a fixed distance of 10 cm was assumed and an estimation made of the amount of time that personnel would carry out these activities (7 h a day, over 10 days). The exposure was calculated to be 420 mSv, as given in Table 2. In the unlikely event that one person worked so closely to the MS at the ‘‘worst’’ angle and for the full period, their absorbed dose was still within the UK legal limits for the year (see Appendix A). 5.3. Radiation safety during lander integration The Beagle 2 lander was encased in its parachute and airbag material and integrated into the probe. This process involved non-radiation personnel working closely round the lander for long periods of time. The worst case value of radiated flux was used, which assumed no shielding from the lander structure, personnel were at a fixed distance of 20 cm and an estimation made of the amount of time that personnel would carry out these activities. The exposure was calculated to be 147 mSv, as given in Table 2, within the UK legal requirements (see Appendix A). The exposure for further activities can similarly be calculated, right up until launch. Table 2 shows the results
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Table 2 Beagle 2 personnel radiation exposure based on continuous exposure at a fixed body distance from MS, for different stages of the build Team
UL Astrium
TLS Astrium
Starsem
Task
Hours per day
Days
Total exposure Based on body Estimated time (h) distance of source dose (cm) (mSv/h)
Biological absorbed dose
Per task mSv
mSv 0.048 0.42 0.147 0.02352 0.02352
PAW integration Lander closeout Probe closeout TLS integration Launch site integration Arming
8 7 7 7 7
1 10 14 14 14
8 70 98 98 98
10 10 20 50 50
6 6 1.5 0.24 0.24
48 420 147 23.52 23.52
7
7
49
100
0.06
2.94 0.00294 Total exposure 0.66498
Spacecraft work Launch site integration Launch site work
7 7
14 20
98 140
50 50
0.24 0.24
7
60
420
100
0.06
Launch site work Guard
7 12
84 84
588 1008
100 100
0.06 0.06
23.52 33.6
0.02352 0.0336
25.2 0.0252 Total exposure 0.08232 35.28 60.48
0.03528 0.06048
of the radiation exposure calculations for the various activities for Beagle 2. 5.4. Radiation safety during probe transport At some stage in the build it will be necessary to transport the probe to be attached onto the carrier spacecraft or integrated onto its launcher. The safety requirements for transport require that the absorbed dose is sufficiently low to allow non-radiation personnel to work safely with the package. The IAEA Regulations For The Safe Transport Of Radioactive Material 1996 [18] require the dose rate to be less than 100 mSv/h at 10 cm from the outer surface of the unpackaged item, i.e. at 10 cm from the surface of the probe. The levels for Beagle 2 were calculated for several points round the probe, as shown in Fig. 6, none of which exceeded 1 mSv/h at 10 cm, well within the limits required. Beagle 2 was transported within its own dedicated transport container within a standard ISO container. 6. Structure and component integrity Spacecraft electronics are susceptible to radiation damage and hence radiation hardness testing is one of the many environmental tests that space instruments and components may have to undergo [15] depending on the expected mission environment. It is also necessary to consider the spacecraft structure tolerance to radiation. The external radiation levels, from solar and cosmic protons and neutrons, were calculated for Mars Express to amount to 6 krad [19] over the mission lifetime and it was required that components be radiation hard to 12 krad.
Fig. 6. Schematic of the Beagle 2 probe showing the position and direction of the Mo¨ssbauer instrument and values of radiative flux in mSv/h at 10 cm from the surface of the probe.
The proximity of many of the PAW instruments to the instrument radiation sources meant that the internal radiation had to be added to the external radiation levels to work out if the components were sufficiently radiation hard to withstand the total expected radiation level. Calculations, using the equations in Section 4, were made for the electronic components on the PAW and for the optical filters on the camera, as well as for the common electronics boards. The calculations all showed that the internal radiation levels were sufficiently small (of order 15 rad at the nearest components) that they added very little to the external radiation levels. The other area of potential concern was the tolerance of the spacecraft structure to the radiation levels. Calculations showed that the various layers of the lander structure absorbed about 0.7 krad from the MS source while the
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airbags absorbed 30 rad, both at the 692 keV energy, the levels from the 122 keV emission being much smaller. As there was no shielding in front of the XRS sources and no other components in its path, all the radiation was absorbed in the lander structure: 5 krad was absorbed in the first 75 mm carbon fibre layer (mainly from the 55Fe radiation) and 6 krad was absorbed in the next 1 cm layer of aluminium honeycomb (mainly from the 109Cd radiation). None of these radiation doses, given the materials and the absorption depths in the materials, were sufficiently high to give concern.
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Table 3 Reduction of sensitivity of XRS due to background radiation from MS 57 Co source Element
Detection limit No
Si K Zn Rb Zr Y Nb
57
4.05 0.06 85 31 38 38 35
Co
Unit With 30.70 0.25 1381 336 257 322 302
57
Co % % ppm ppm ppm ppm ppm
7. Effects on scientific performance As discussed above radiation may damage electronic components, however, it may also manifest itself as degraded performance of an instrument, for example, CCD detectors will exhibit poorer spectral resolution with dose [17]. Also where there is another instrument that operates by detecting radiation this ‘‘external’’ radiation may appear as unwanted background. In the early stage of development of Beagle 2, measurements were taken of XRS spectra with a 57Co source in close proximity. The results showed that there was a large increase in the background of the XRS spectrum which would reduce its sensitivity, as shown in Fig. 7. It was clear that many of the spectral peaks were swamped by the scatter background, destroying the capability of the XRS to measure some of the elements and reducing its sensitivity to other elements. Table 3 shows the calculated detection limits for various elements in a granite reference sample, showing the reduced sensitivity from when there is no 57Co present to when the source is in place. It can be seen that the sensitivity drops by a factor from 4 to 8 for the major elements and by about 10 for the trace elements. The Compton scatter from the MS is clearly seen in Fig. 7 and its scatter angle can be calculated from the Compton edge (calculated to be 79.51). From measurements with and without the XRS sources in place, it was discovered that most of the added
8. Summary It is clear from the experience of Beagle 2 that when dealing with radioactive sources in planetary instrumentation hazards and effects and possible solutions should be considered early on in the design phase of the spacecraft, taking into account all energies from all sources. Particular issues to consider include:
10000 Ag K
Ec
Au K
100
Compton Edge
with Co-57
Co-57 122 keV photopeak
10
no Co-57
1 0
20
personnel safety at all stages of the programme, radiation damage to materials and components, including external radiation dose, effects on performance of scientific instruments.
The following steps can be taken to reduce or eliminate these hazards:
1000
counts
background arose from the 57Co 122 keV radiation being scattered off the XRS source capsules, these being high Z and high scatter materials in close proximity to the XRS detector. An estimation of the angle from drawings of the 57 Co source to the XRS sources to the XRS detector was made and the scatter angle was found to be 801751, which corresponded well with the angle calculated using the Compton edge energy above. With the addition of Ta shielding round the MS source, within the instrument body, the leakage radiation was reduced considerably so that there was no noticeable background in the XRS spectra from the MS.
40
60
80
100
120
140
energy / keV
Fig. 7. XRS spectrum of a granite sample showing enhanced background from MS Co-57.
keep source strengths as low as possible that still allow the science to be performed, minimise time spent in proximity to the sources, carry out dry runs to practise manoeuvres to minimise time, design ease of access of source insertion and instrument integration, maximise distance to the sources, insert shielding close to sources, use graded shielding where necessary.
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In practice it is also advisable to
make worst case calculations,
but more importantly to
take measurements with a properly calibrated instrument.
Acknowledgements We would like to acknowledge the support of PPARC for funding the work at the University of Leicester. Discussions with John Scott, Radiation Safety Officer of the University of Leicester were particularly helpful. Thanks also to the drawing office team at Astrium for providing detailed drawings of the lander and probe. Appendix A. Relevant radiation regulations The regulations relevant to Beagle 2 and the UK, concerning the radiation dose are: The UK Ionising Radiation Regulations 1999, Schedule 1, Regulations 6(1) and 13(3) state that ‘‘work with ionising radiation shall not be required to be notified in accordance with regulation 6 when y the apparatus does not under normal operating conditions cause a dose rate of more than 1 mSv/h at a distance of 0.1 m from any accessible surface’’ [20]. The UK Ionising Radiation Regulations 1999, Schedule 4, Regulation 11 state that ‘‘the limit on effective dose for any person other than an employee or trainee, including any person below the age of 16, shall be 1 mSv in any calendar year’’ [20]. A controlled access area is designated when the dose rate is more than 7 mSv/h at a distance of 1 m. UK Regulations for the Safe Transport of Radioactive Material 1996 state that for an excepted package the radiation level at 10 cm from the external surface of any unpackaged instrument shall not exceed 0.1 mSv/h [21].
The IAEA Regulations For The Safe Transport Of Radioactive Material 1996 require that for an excepted package the biological absorbed dose must be o100 mSv/h at 10 cm from the external surface of the unpackaged instrument [18].
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