- Email: [email protected]
A detector telescope with charge coupled devices for particle dosimetry in space
Radiation Measurements. Vol. 28, Nos I-6, pp. 211-216. 1997
Pergamon Pll: S 1350-4487(97)00070-X
O 1997ElsevierScienceLid Printedin GreatBritain.All rightsreserved 1350-4487/97 $17.00+ 0.00
A DETECTOR TELESCOPE WITH CHARGE COUPI,ED DEVICES FOR PARTICLE DOSIMETRY IN SPACE
J. U. SCHOTT, M. M. MEIER AND K. STRAUCH DLR Institute of Acrospac,¢ Medicine, D-51140 K61n ABSTRACT A telescopic device with charge coupled devices (CCDs) for particle dosime~), in space has been developed. Data on ionization events of enecgetic particles passing the CCDs are processed in an image analyzing system of a PC. As 'Charged Particle Telescope' (CHAPAT), the equipment was flown on the russian space station MIR during the EUROMIR mission in 1995. The response of the CCDs to various charged particles and methods for the discrimination of heavy particles in CCDs are discussed. First resuRs of a correlation of temporal particle fluxes to the actual orbital parameters of MIR clearly identify passages through the South Atlantic Anomaly (SAA). KEYWORDS High LET particles; particle discrimination, charge coupled devices; space radiation; South Atlantic Anomaly; radiobiology. INTRODUCI'ION With the advent of manned space flight, radiation risk assessment entered new dimensions. It encountered the energetic heavy particle component of the space radiation field, a radiation quality being absent and hence without established risk estimates on earth. The lack of adequate radiobiological _,:lata for risk assessments enforced experiments on radiobiological effects of single heavy ions, ground based and in space. First investigations on the radiobiological effects of single heavy ions of the near earth radiation field on biological samples were performed in the Biostack experiments in the 1970s (Biicker et el., 1973). They clearly have correlated severe biological damage in resting states of biological systems, hit or closely passed by a single heavy ion. However, radiobiological data useful for risk estimates for human in space should be obtained from biological systems as close as possible to humans. One step towards this requirement is the investigation of metabolizing (growing. or moving, or growing and moving) objects on a high organization level. Experiments with those objects, seedlings, larvae etc., require on-line information on the trajectory of single particles and the detection of the position of the biological target for correlation. Replacing solid state nuclear track detectors by charge coupled devices (CCDs) gives rise to gain any information necessary for radiobiologieal analysis (Schott, 1988). With a spatial resolution in the order of the size of a pixei element (10 gin) for particle detection and optical outlining of biological samples growing on the surface of the light sensitive field of the CCD, seedlings of arabidopsis thaliana have been investigated. The development of an apex tumor has been obtained in a seedling hit by a single Pb ions of 29 MeV/amu from the GANIL accelerator (Schott et el., 1992; Schott, 1993a and b). The CHAPAT is a first step for the investigation of the radiobiological effectiveness of single heavy ions onto metabolizing objects in space environment. In the EUROMIR'95 mission, its operation is limited to physical dasimetry, only.
211
PROCEEDINGS OF THE 18TH INTERNATIONAL CONFERENCE
212
METHODS In dosimetric experiments in space additional information on particles are required for the correlation with radiobiological findings. They are the direction of the particle and, rather than its linear energy transfer (LET), its charge and energy. The later quantities are more appropriate for the quantification of radiation damage obtained in ground based experiments.
The broad range of charges and energies of particles in space requires more than one approach for particle detection and discrimination in the CCDs. 1) A telescopic arrangement of three CCDs, operating in coincidence, localizes the trajectory of single particles with an accuracy of 10 tam, and yields the direction with an accuracy better than 0.5 degrees. The change of brightness of hit pixels along the path of the particle yields information of Z and E. For the calibration of pixel brightness against energy deposition, CCDs have been exposed to a 12 kV X-ray spectrum from a Cu-anode, and to 10 MeV protons of normal incidence. Figure 1 shows the frequency density of energy deposited from 10 MeV protons in the depletion volume. From both calibrations the thickness of the depletion volume of 1.2 inn has been derived. 2,0e-2
Protons '> v
10 MeV
1,5e-2
~ 1,0e-2 5,0e-3 1,
O,Oe+O 5
10
15
20
25
30
Energy deposited in pixel (keV)
Figure 1. Frequency density of energy deposited in CCD pixels by 10 MeV protons.
2) Compared to the use of CCDs in the optical field, exposures to particles are extremely short termed, high densities of secondary electrons occur. For high LET particles, saturation effects and lost of charge carriers result in limited resolution and in non-linearity of the response against LET. For the discrimination of particles with Z>I5 and E>20 MeV/amu, a new method has been introduced.
Figure 2.
Ionizing events of a single Xe-ion of 42 MeV/amu detected in a CCD behind a 4.2mg/cm2 thick Carbon foil, spaced 800 lain in front of the sensitive pixei matrix. The bright cluster in the center is due to the traversal of the ion, the halo of dots is caused by secondary electrons, emitted from the Carbon foil.
PROCEEDINGS OF THE 18TH INTERNATIONAL CONFERENCE
213
Charge and energy of particles can be determined from the number and the angular distribution of secondary electrons, being emitted along the traversal of the particle through absorbing material. A thin foil of carbon, closely spaced in front of the sensitive area of the CCD is used as such an absorber. A heavy ion, passing through the foil yield a cloud of electrons, leaving the foil at the surface facing the CCD. Both, heavy particle and electrons hit the CCD and get detected, spatially resolved. The number and angular distribution of electrons being detected is modulated compared to the initial distribution of secondary electrons, due to transport interactions in the foil along their path between liberation and the depletion volume of the CCD. However, a quantitative analysis of number and angular distribution of electrons in the energy range being detected yields experimental data for particle identification in a model (Meier et al.,1996). Figure 2 shows the picture of the ionization events of one 42 MeV/amu Xe-ion and its electron shower in the 4x6 mm large CCD pixel matrix. Figure 3 shows the angular distribution of electrons detected (average of 16 single events).
Xe 42 M e V / a m u
f-
o
2
0.
r10
1
o 20
40
60
80
J [dog] Figure 3. Angulardistribution of electrons, averagedover 16 single42 MeV/amuXe ¢vonts. Dots are experimental data, the curve is the result of simulation,reported elsewhere in this proceeding (Meier, 1996). Thus, the determination of particle parameters is made possible simply by the evaluation of spatially resolved detection of electrons liberated by the particle. For the EUROMIR'95 mission a charged particle telescope (CHAPAT) has been developed. It consists of a telescope head and a PC for acquisition and processing of the radiation data. The telescope head houses the sensors and electronics for the conversion of the CCD data into a CCIR video signals. For radiation sensing. 3 CCD frame transfer sensors of type FT800 without cover glasses are mounted directly on top of each other. The uppermost CCDs are carried by special p r i n t l ~ d s . CCDs with cut pins are mounted into a cut offofthe boards in such a way, that the board together with the CCD and some surface mounted electronics is not thicker than the case of the CCD itself. This permits closest positioning of the 3 CCDs, limited by the dimensions of the case, only. The aperture of the telescope reaches 3.9 sr for particle detection in two CCDs and 3.0 sr in three CCDs. Electrically, the telescope head is operated as a three channel (RGB) video camera. The necessary three modules, type FTM$00, for video signal processing (two boards, each) are attached in close vicinity of the CCDs, together with an operational board for camera parameter setting, temperature monitoring and fan operation. Figure 4 shows the telescope head. The opened compartment makes visible the 6 processing boards, the operational board and the fan on the cover plate. The CCDs are stacked in the smaller (closed) compartment behind a 25 ~m thick Ti window. Figure 5 shows the results of an analysis of the mass absorption. In order to detect even low energetic particles in at least m 2a:116-I
P R O C E E D I N G S OF T H E 18TH I N T E R N A T I O N A L C O N F E R E N C E
214
two fold coincidence, the uppermost sensor has been used without ceramic housing behind the substrate. The sensors are able to detect protons in the energb, range from 0.8 to 70 MeV, the telescope from 0.8 to 15 MeV without, 15 to 36 MeV in two fold, and 36 to 70 MeV in three fold coincidence for particles of normal incidence. For particles, hitting the CCDs with an inclination, the read out signals are increased due to the longer path length in the depletion volume. For the measurement and analysis of angular spectra of electrons emitted from rare events of particles with high atomic number and enerKy, a 25 pm thick Ta foils is positioned in a distance of 800 pm from the sensing pixel matrix on any of the CCDs.
Figure 4. Charged particle telescope, telescope head. Thickness(g/cm 2)
/ Titan
CCD~
|
CCDa
.
.
.
.
.
/
t
,
I
o,oll
iii:1iiiiiiiiiiiiiiiiii:;i:iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii:ii: O,Oll iiiiiiii;;;iiii:ii 0,0005 m
CCD2
,rrr
0,15 0,011 0,0005 0,15 0,4
0,011 0,0005 0,15 0,4
titan pixellayer silicium ceramics
Figure 5. Set up of CCDs and mass absorption in the telescope.
The operation of the telescope head is controlled by a PC with slot boards for on-line image analysis (AFP and PRIMA SPEED, LEUTRON VISION) and an I/O board. It sets the parameters for black level and gain to the modules and activates software programs for automatic operational sequences, data analysis, logical decisions, and data reduction and storage.
PROCEEDINGS OF THE 18TH INTERNATIONALCONFERENCE
215
The low fluxes of particles expected in space, especially with high atomic numbers makes continuos operation necessary. With 8 bit digitizing of the frames, ,~_ta rates of 25 Mbyte/s are obtained. The restricted capability for data storage and linking to ground with space experiments requires to reduce the data. The data of coincident frames are analyzed pixelwise on-line for particle event detection. Frames with an event get stored either as full RGB frame, or as small R, G and B pixel matrixes around the single events, only. Criterion for on-line particle detection is the comparison of the digitized pixel value with a threshold, being defined automatically from statistical analysis of the noise distribution done in space environment. In order to discriminate particle triggering from pixels potentially damaged from previous high LET processes, the threshold for triggering is set on the difference of pixel values of the life frame to the previous frame (difference frame). Data packages of 150 Kbytes are made available in near real time on ground for status control and setting of automatic scanning programs on the system or for changing the identified threshold or other parameters.
THE EXPERIMENT As part of the dosimetric experiment "Radiation Health During Prolonged Spaceflight (DOM)' of the EUROMIR'95 mission (Principal Investigator: G. Reitz), CHAPAT was set up inside one of the modules (Kristal) of the russian space station MIR in an orbit of 406 km attitude and 52 degrees of inclination. CHAPAT operated from Sept. 10, 1995 to Nov. 10, 1995. The activation of the PC requires to setting date and time, and software parameters, and to choose a program out of a menu for automatic operation. All experimental data and a record of the inputs set by crew members get stored on DAT tapes for recovery after the mission, some of them have been recorded on diskette for downlinking through the MIR telemetric data system (MIPS). Diskettes and DAT tapes have been exchanged by a crew member on request by the computer. For the definition of a threshold for automatic particle event recognition, CHAPAT started with an on board analysis of the noise distribution of the pixels in space environment. As long as the instrument does not get exposed in the radiation belt of the South Atlantic Anomaly (SPA) it is assumed, that the contribution of particle events to the noisy frame can be neglected. Due to the fact, that CHAPAT has no information on actual orbital parameters, noise data of 25 orbits have been analyzed for temporal fluctuations of the number of high level noisy pixels. Orbits in which strong deviations are obtained in the normal distribution are considered to be orbits touching the SAA, the data of which are rejected from further analysis. The threshold was set such, that the expected trigger rate is not bigger than 10.3 per frame for orbits not touching the SAA. The results of this analysis, together with the data of some noisy frames have been linked to ground. With the automatically set threshold particle events have been triggered under different conditions of coincidence. Fully sized R, G and B frames with particle events have been stored together with the difference frames. For a long termed search for high LET particles, these measurements have been repeated with different thresholds, manually set.
RESULTS CHAPAT did withstand the mechanical and electrical stresses of spaceflight. It has taken about 10 Gbyte of data on DAT tapes during the EUROMIR'95 mission. The tapes have been recovered after the mission for evaluation. The first evaluation of data concentrates on the trigger rates for particle hits detected in a single CCD. Figure 6 shows the rates during three orbits during Sept. 12, 1995, with a strong increase of the frequency of particle events at 8:1 l, 9:41 and 11:21 Moscow local time. These times coincide with the orbital parameters for two orbits touching and one orbit fully traversing the SAA. As expected, coincident particle events have been obtained, mainly in the uppermost and the middle placed CCD. An analysis of the coincident data for frequency and angular distribution of particles is in progress.
216
PROCEEDINGS OF THE 18TH INTERNATIONAL CONFERENCE 14
CHAPAT
12
EUROMIR'95
e-
"~
S e p t . 12, 1 9 9 5
10
@
o
._~ J=
6
@
n
2 v
. . . .
7:46
II III II
I . . . .
8:36
1"
9:26
,I I i ! 1 ! 10:16
11:06
11:56
M o s c o w local time
Figure 6. CHAPATtrigger rate in the course of three orbits touching the South Atlantic Anomaly (SAA). (The lack of data from 9:46 to 10:22 is due to the deactivation of on-line particle search during transfer of data to magnetic tape.)
CONCLUSION CHAPAT was a first step for a CCD particle telescope to go to space. Although a complete evaluation of data is not yet done, CHAPAT demonstrates, that CCDs are useful electronic devices for the detection of single particles in space. Their high spatial and temporal resolution and easy handling favors them for use in investigations of radiobiological effects by single heavy ions of the space radiation field in metabolizing biological objects. Admewledgment-For the provision of beamtime for the calibration of CCDs we thank GANIL, GSI, PSI and the Universities of Bonn and Frankfurt am Main (IKF).
REFERENCES
Biicker H., Horneck G., Reinholz E., Riither W., Graul E.H., Planel H., Soleihavoup J.P., Cuer P., Kaiser R., Massue J.P., Pfohl R., Enge W,, Bartholom~i W.P., Beaujean R., Fukui K., Allkofer O.C., Heinrich W., Benton E.V., Schopper E., Henig G., Schott J.U., Francois H., Portal G., Kiihn H., Harder D., Wollenhaupt H. and Bowman G. (1973) BIOSTACK Experiment; NASA SP-330, 25-1. Schott J.U. (1988) Charge coupled devices (CCDs) - A detector system for particles with time resolution and local assignment with particle trajectories. Nucl. Tracks Radiat. Meas. 15, Nos 14, 81-89. Schott J.U. (1993) Time Resolving Detector Systems for Radiobiologlcal Investigations of Effects of Single Heavy Ions. In Biological Effects and Physics of Solar and Galactic Cosmic Radiation, NA TO AS] Series A, Vol. 243B, 153-163. Schott J.U., Kranz A.R., Gartenbach K. and Zimmermann M. (1992) Investigation of Single Particle Effects in Active Metabolizing Seedlings of Arabidopsis thaliana. In Nouvelles du GANIL, No. 42, 7-9. Schott J.U. (1993) Time Resolving Detector Systems: Charge Coupled Devices in Studies of Single Particle Events, Tracks Radiat. Meas. 22, Nos 1-4, 73-79. Meier M.M., Schott J.U. and Strauch K. (1996) Detection of Single Heavy Swirl Ions with a System Based on Charge Coupled Devices, to be published elsewhere in these Proceedings.
Pergamon Pll: S 1350-4487(97)00070-X
O 1997ElsevierScienceLid Printedin GreatBritain.All rightsreserved 1350-4487/97 $17.00+ 0.00
A DETECTOR TELESCOPE WITH CHARGE COUPI,ED DEVICES FOR PARTICLE DOSIMETRY IN SPACE
J. U. SCHOTT, M. M. MEIER AND K. STRAUCH DLR Institute of Acrospac,¢ Medicine, D-51140 K61n ABSTRACT A telescopic device with charge coupled devices (CCDs) for particle dosime~), in space has been developed. Data on ionization events of enecgetic particles passing the CCDs are processed in an image analyzing system of a PC. As 'Charged Particle Telescope' (CHAPAT), the equipment was flown on the russian space station MIR during the EUROMIR mission in 1995. The response of the CCDs to various charged particles and methods for the discrimination of heavy particles in CCDs are discussed. First resuRs of a correlation of temporal particle fluxes to the actual orbital parameters of MIR clearly identify passages through the South Atlantic Anomaly (SAA). KEYWORDS High LET particles; particle discrimination, charge coupled devices; space radiation; South Atlantic Anomaly; radiobiology. INTRODUCI'ION With the advent of manned space flight, radiation risk assessment entered new dimensions. It encountered the energetic heavy particle component of the space radiation field, a radiation quality being absent and hence without established risk estimates on earth. The lack of adequate radiobiological _,:lata for risk assessments enforced experiments on radiobiological effects of single heavy ions, ground based and in space. First investigations on the radiobiological effects of single heavy ions of the near earth radiation field on biological samples were performed in the Biostack experiments in the 1970s (Biicker et el., 1973). They clearly have correlated severe biological damage in resting states of biological systems, hit or closely passed by a single heavy ion. However, radiobiological data useful for risk estimates for human in space should be obtained from biological systems as close as possible to humans. One step towards this requirement is the investigation of metabolizing (growing. or moving, or growing and moving) objects on a high organization level. Experiments with those objects, seedlings, larvae etc., require on-line information on the trajectory of single particles and the detection of the position of the biological target for correlation. Replacing solid state nuclear track detectors by charge coupled devices (CCDs) gives rise to gain any information necessary for radiobiologieal analysis (Schott, 1988). With a spatial resolution in the order of the size of a pixei element (10 gin) for particle detection and optical outlining of biological samples growing on the surface of the light sensitive field of the CCD, seedlings of arabidopsis thaliana have been investigated. The development of an apex tumor has been obtained in a seedling hit by a single Pb ions of 29 MeV/amu from the GANIL accelerator (Schott et el., 1992; Schott, 1993a and b). The CHAPAT is a first step for the investigation of the radiobiological effectiveness of single heavy ions onto metabolizing objects in space environment. In the EUROMIR'95 mission, its operation is limited to physical dasimetry, only.
211
PROCEEDINGS OF THE 18TH INTERNATIONAL CONFERENCE
212
METHODS In dosimetric experiments in space additional information on particles are required for the correlation with radiobiological findings. They are the direction of the particle and, rather than its linear energy transfer (LET), its charge and energy. The later quantities are more appropriate for the quantification of radiation damage obtained in ground based experiments.
The broad range of charges and energies of particles in space requires more than one approach for particle detection and discrimination in the CCDs. 1) A telescopic arrangement of three CCDs, operating in coincidence, localizes the trajectory of single particles with an accuracy of 10 tam, and yields the direction with an accuracy better than 0.5 degrees. The change of brightness of hit pixels along the path of the particle yields information of Z and E. For the calibration of pixel brightness against energy deposition, CCDs have been exposed to a 12 kV X-ray spectrum from a Cu-anode, and to 10 MeV protons of normal incidence. Figure 1 shows the frequency density of energy deposited from 10 MeV protons in the depletion volume. From both calibrations the thickness of the depletion volume of 1.2 inn has been derived. 2,0e-2
Protons '> v
10 MeV
1,5e-2
~ 1,0e-2 5,0e-3 1,
O,Oe+O 5
10
15
20
25
30
Energy deposited in pixel (keV)
Figure 1. Frequency density of energy deposited in CCD pixels by 10 MeV protons.
2) Compared to the use of CCDs in the optical field, exposures to particles are extremely short termed, high densities of secondary electrons occur. For high LET particles, saturation effects and lost of charge carriers result in limited resolution and in non-linearity of the response against LET. For the discrimination of particles with Z>I5 and E>20 MeV/amu, a new method has been introduced.
Figure 2.
Ionizing events of a single Xe-ion of 42 MeV/amu detected in a CCD behind a 4.2mg/cm2 thick Carbon foil, spaced 800 lain in front of the sensitive pixei matrix. The bright cluster in the center is due to the traversal of the ion, the halo of dots is caused by secondary electrons, emitted from the Carbon foil.
PROCEEDINGS OF THE 18TH INTERNATIONAL CONFERENCE
213
Charge and energy of particles can be determined from the number and the angular distribution of secondary electrons, being emitted along the traversal of the particle through absorbing material. A thin foil of carbon, closely spaced in front of the sensitive area of the CCD is used as such an absorber. A heavy ion, passing through the foil yield a cloud of electrons, leaving the foil at the surface facing the CCD. Both, heavy particle and electrons hit the CCD and get detected, spatially resolved. The number and angular distribution of electrons being detected is modulated compared to the initial distribution of secondary electrons, due to transport interactions in the foil along their path between liberation and the depletion volume of the CCD. However, a quantitative analysis of number and angular distribution of electrons in the energy range being detected yields experimental data for particle identification in a model (Meier et al.,1996). Figure 2 shows the picture of the ionization events of one 42 MeV/amu Xe-ion and its electron shower in the 4x6 mm large CCD pixel matrix. Figure 3 shows the angular distribution of electrons detected (average of 16 single events).
Xe 42 M e V / a m u
f-
o
2
0.
r10
1
o 20
40
60
80
J [dog] Figure 3. Angulardistribution of electrons, averagedover 16 single42 MeV/amuXe ¢vonts. Dots are experimental data, the curve is the result of simulation,reported elsewhere in this proceeding (Meier, 1996). Thus, the determination of particle parameters is made possible simply by the evaluation of spatially resolved detection of electrons liberated by the particle. For the EUROMIR'95 mission a charged particle telescope (CHAPAT) has been developed. It consists of a telescope head and a PC for acquisition and processing of the radiation data. The telescope head houses the sensors and electronics for the conversion of the CCD data into a CCIR video signals. For radiation sensing. 3 CCD frame transfer sensors of type FT800 without cover glasses are mounted directly on top of each other. The uppermost CCDs are carried by special p r i n t l ~ d s . CCDs with cut pins are mounted into a cut offofthe boards in such a way, that the board together with the CCD and some surface mounted electronics is not thicker than the case of the CCD itself. This permits closest positioning of the 3 CCDs, limited by the dimensions of the case, only. The aperture of the telescope reaches 3.9 sr for particle detection in two CCDs and 3.0 sr in three CCDs. Electrically, the telescope head is operated as a three channel (RGB) video camera. The necessary three modules, type FTM$00, for video signal processing (two boards, each) are attached in close vicinity of the CCDs, together with an operational board for camera parameter setting, temperature monitoring and fan operation. Figure 4 shows the telescope head. The opened compartment makes visible the 6 processing boards, the operational board and the fan on the cover plate. The CCDs are stacked in the smaller (closed) compartment behind a 25 ~m thick Ti window. Figure 5 shows the results of an analysis of the mass absorption. In order to detect even low energetic particles in at least m 2a:116-I
P R O C E E D I N G S OF T H E 18TH I N T E R N A T I O N A L C O N F E R E N C E
214
two fold coincidence, the uppermost sensor has been used without ceramic housing behind the substrate. The sensors are able to detect protons in the energb, range from 0.8 to 70 MeV, the telescope from 0.8 to 15 MeV without, 15 to 36 MeV in two fold, and 36 to 70 MeV in three fold coincidence for particles of normal incidence. For particles, hitting the CCDs with an inclination, the read out signals are increased due to the longer path length in the depletion volume. For the measurement and analysis of angular spectra of electrons emitted from rare events of particles with high atomic number and enerKy, a 25 pm thick Ta foils is positioned in a distance of 800 pm from the sensing pixel matrix on any of the CCDs.
Figure 4. Charged particle telescope, telescope head. Thickness(g/cm 2)
/ Titan
CCD~
|
CCDa
.
.
.
.
.
/
t
,
I
o,oll
iii:1iiiiiiiiiiiiiiiiii:;i:iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii:ii: O,Oll iiiiiiii;;;iiii:ii 0,0005 m
CCD2
,rrr
0,15 0,011 0,0005 0,15 0,4
0,011 0,0005 0,15 0,4
titan pixellayer silicium ceramics
Figure 5. Set up of CCDs and mass absorption in the telescope.
The operation of the telescope head is controlled by a PC with slot boards for on-line image analysis (AFP and PRIMA SPEED, LEUTRON VISION) and an I/O board. It sets the parameters for black level and gain to the modules and activates software programs for automatic operational sequences, data analysis, logical decisions, and data reduction and storage.
PROCEEDINGS OF THE 18TH INTERNATIONALCONFERENCE
215
The low fluxes of particles expected in space, especially with high atomic numbers makes continuos operation necessary. With 8 bit digitizing of the frames, ,~_ta rates of 25 Mbyte/s are obtained. The restricted capability for data storage and linking to ground with space experiments requires to reduce the data. The data of coincident frames are analyzed pixelwise on-line for particle event detection. Frames with an event get stored either as full RGB frame, or as small R, G and B pixel matrixes around the single events, only. Criterion for on-line particle detection is the comparison of the digitized pixel value with a threshold, being defined automatically from statistical analysis of the noise distribution done in space environment. In order to discriminate particle triggering from pixels potentially damaged from previous high LET processes, the threshold for triggering is set on the difference of pixel values of the life frame to the previous frame (difference frame). Data packages of 150 Kbytes are made available in near real time on ground for status control and setting of automatic scanning programs on the system or for changing the identified threshold or other parameters.
THE EXPERIMENT As part of the dosimetric experiment "Radiation Health During Prolonged Spaceflight (DOM)' of the EUROMIR'95 mission (Principal Investigator: G. Reitz), CHAPAT was set up inside one of the modules (Kristal) of the russian space station MIR in an orbit of 406 km attitude and 52 degrees of inclination. CHAPAT operated from Sept. 10, 1995 to Nov. 10, 1995. The activation of the PC requires to setting date and time, and software parameters, and to choose a program out of a menu for automatic operation. All experimental data and a record of the inputs set by crew members get stored on DAT tapes for recovery after the mission, some of them have been recorded on diskette for downlinking through the MIR telemetric data system (MIPS). Diskettes and DAT tapes have been exchanged by a crew member on request by the computer. For the definition of a threshold for automatic particle event recognition, CHAPAT started with an on board analysis of the noise distribution of the pixels in space environment. As long as the instrument does not get exposed in the radiation belt of the South Atlantic Anomaly (SPA) it is assumed, that the contribution of particle events to the noisy frame can be neglected. Due to the fact, that CHAPAT has no information on actual orbital parameters, noise data of 25 orbits have been analyzed for temporal fluctuations of the number of high level noisy pixels. Orbits in which strong deviations are obtained in the normal distribution are considered to be orbits touching the SAA, the data of which are rejected from further analysis. The threshold was set such, that the expected trigger rate is not bigger than 10.3 per frame for orbits not touching the SAA. The results of this analysis, together with the data of some noisy frames have been linked to ground. With the automatically set threshold particle events have been triggered under different conditions of coincidence. Fully sized R, G and B frames with particle events have been stored together with the difference frames. For a long termed search for high LET particles, these measurements have been repeated with different thresholds, manually set.
RESULTS CHAPAT did withstand the mechanical and electrical stresses of spaceflight. It has taken about 10 Gbyte of data on DAT tapes during the EUROMIR'95 mission. The tapes have been recovered after the mission for evaluation. The first evaluation of data concentrates on the trigger rates for particle hits detected in a single CCD. Figure 6 shows the rates during three orbits during Sept. 12, 1995, with a strong increase of the frequency of particle events at 8:1 l, 9:41 and 11:21 Moscow local time. These times coincide with the orbital parameters for two orbits touching and one orbit fully traversing the SAA. As expected, coincident particle events have been obtained, mainly in the uppermost and the middle placed CCD. An analysis of the coincident data for frequency and angular distribution of particles is in progress.
216
PROCEEDINGS OF THE 18TH INTERNATIONAL CONFERENCE 14
CHAPAT
12
EUROMIR'95
e-
"~
S e p t . 12, 1 9 9 5
10
@
o
._~ J=
6
@
n
2 v
. . . .
7:46
II III II
I . . . .
8:36
1"
9:26
,I I i ! 1 ! 10:16
11:06
11:56
M o s c o w local time
Figure 6. CHAPATtrigger rate in the course of three orbits touching the South Atlantic Anomaly (SAA). (The lack of data from 9:46 to 10:22 is due to the deactivation of on-line particle search during transfer of data to magnetic tape.)
CONCLUSION CHAPAT was a first step for a CCD particle telescope to go to space. Although a complete evaluation of data is not yet done, CHAPAT demonstrates, that CCDs are useful electronic devices for the detection of single particles in space. Their high spatial and temporal resolution and easy handling favors them for use in investigations of radiobiological effects by single heavy ions of the space radiation field in metabolizing biological objects. Admewledgment-For the provision of beamtime for the calibration of CCDs we thank GANIL, GSI, PSI and the Universities of Bonn and Frankfurt am Main (IKF).
REFERENCES
Biicker H., Horneck G., Reinholz E., Riither W., Graul E.H., Planel H., Soleihavoup J.P., Cuer P., Kaiser R., Massue J.P., Pfohl R., Enge W,, Bartholom~i W.P., Beaujean R., Fukui K., Allkofer O.C., Heinrich W., Benton E.V., Schopper E., Henig G., Schott J.U., Francois H., Portal G., Kiihn H., Harder D., Wollenhaupt H. and Bowman G. (1973) BIOSTACK Experiment; NASA SP-330, 25-1. Schott J.U. (1988) Charge coupled devices (CCDs) - A detector system for particles with time resolution and local assignment with particle trajectories. Nucl. Tracks Radiat. Meas. 15, Nos 14, 81-89. Schott J.U. (1993) Time Resolving Detector Systems for Radiobiologlcal Investigations of Effects of Single Heavy Ions. In Biological Effects and Physics of Solar and Galactic Cosmic Radiation, NA TO AS] Series A, Vol. 243B, 153-163. Schott J.U., Kranz A.R., Gartenbach K. and Zimmermann M. (1992) Investigation of Single Particle Effects in Active Metabolizing Seedlings of Arabidopsis thaliana. In Nouvelles du GANIL, No. 42, 7-9. Schott J.U. (1993) Time Resolving Detector Systems: Charge Coupled Devices in Studies of Single Particle Events, Tracks Radiat. Meas. 22, Nos 1-4, 73-79. Meier M.M., Schott J.U. and Strauch K. (1996) Detection of Single Heavy Swirl Ions with a System Based on Charge Coupled Devices, to be published elsewhere in these Proceedings.