CR-39 used for cosmic ray measurements aboard Spacelab-1

0191-278X/86 $3.00+.00 Pergamon Journals Ltd.

Nuclear Tracks, Vol. 12, Nos I-6, pp. 419-422, 1986. Int. J. Radiat. Appl. Instrum., Part D Printed in Great Britain.

CR-39 USED FOR COSMIC RAY MEASUREMENTS

J.Krause,

R.Beaujean,

ABOARD SPACELAB-I

E.Fischer and W.Enge

Institut f~r Reine und Angewandte Kernphysik University of Kiel, D-2300 Kiel, W.-Germany

ABSTRACT A stack of CR-39 plastic track detectors with a time resolution system was exposed to the cosmic radiation aboard Spacelab-1. The impact time of the incoming particles was determined with an accuracy of about 2 minutes. For each time interval which is correlated to an orbit interval a minimum cutoff rigidity exists. In the energy range 50 150 MeV/nuc 22 "geomagnetically forbidden" particles were found with a rigidity less than the required cutoff value assuming fully stripped ions. These particles hit the stack mainly in the southern hemisphere at local night time. The charge resolution of the used CR-39 is 0.37 charge units for single cone measurements. KEYWORDS CR-39,

Spacelab-1,

cosmic rays,

"magnetically

forbidden" particles

INTRODUCTION Spacelab-1, a joint NASA/ESA mission was the first Spaceshuttle flight of the European Spacelab system from 11/28/1983 - 12/8/1983. The orbit had 57 ° inclination and about 250 km altitude. The Kiel experiment "Isotopic Stack" was designed to measure heavy cosmic ray nuclei with nuclear charge Z~3 in the energy range 10 - 700 MeV/nuc and it was mounted on the pallet which was open to the space. EXPERIMENT

DESCRIPTION

A detailed description of the design and operation of the experiment is given in Oschlies et ai.(1985). The parts 2 - 4 of the stack, consisting of CR-39 (nominally pure, curing cycle 32 hours~ produced by Pershore Mouldings Ltd., Pershore, Worcestershire, England) were treated as follows before installation in the aluminium container. They were outgased in vacuum (I mbar, 72 hours) and then poured in at the edge and pasted with Eccobond 55 Epoxy Adhesive (Emerson and Cumming, B-2431 0evel). POST FLIGHT EXPERIMENTAL

PROCEDURE

For post flight calibration the stack was exposed to a 410 MeV/nuc Fe-56 beam at Lawrence Berkeley Laboratory. After dismantling of the experiment the adhesive edges of the CR-39 stacks were turned off on a lathe and the foils were processed 20 hours in (6±0.1)n-NaOH at (70±0. I)°C. The third part of the stack, fixed directly under the rotating part was scanned under a stereo microscope in the radius range (3.7 - 21.4)cm. RESULTS AND DISCUSSION 749 tracks of stopping cosmic ray particles with energies of 50 - 150 MeV/nuc were found in this volume. The trajectories of only 557 particles could be traced into the rotating part of the stack due to the short etchable length of tracks of lighter particles. 43 of these particles hit the stack in the 12 hours interval before the experiment was powered, 365 particles in the 177.5 hours interval when it was powered and 149 particles in the 58.5 hours interval after switching off the stepping.

419

J. KRAUSE ¢# a l .

420

The impact time of the particles is provided by measuring the turning angle of a track segment in the rotating stack in respect to the track segment 55in the fixed part. The accuracy of this measure50ment is shown in Fig. l, where the deviation of all measured turning angles from the nearest rest time angle is plotted. Most of the particles hit the stack in rest times as the stack was always moved 35 for 4 seconds and then stood still for at least 90 seconds. This figure includes the data obtained -~. 30 from tracks with flat incident angles as well as z 25 data from tracks with azimuth angle parallel to the turning motion. The measurement has a standard deviation of 0.065 degrees whereas the angle 15: difference of two successive rest positions was about 0.34 degrees. With this time resolution it is possible to correlate the particles with orbit 5_ intervals of the shuttle and thus with vertical cutoff rigidity intervals of the earth's magnetic ~9-8-1-6 -~-~ -3-2-1 field (Oschlies et al., 1985). This preliminary Deviation ~O.0226 aegrees~ selection yields 39 particles as candidates to be "magnetically forbidden". Fig.1 Deviation between the To prove this selection charge and energy of the measured alignment posiparticles had to be determined. For charge calition and the nearest bration 101 tracks of the Fe-56 calibration beam rest position. were measured under a semi-automatic optical m i c r o scope with the wellknown conelength versus residual range (dE/dx - E) method. The result is shown in Fig.2. From these data we derived a charge resolution of 0.37 charge units for a single cone and the response curve shown in Fig. 3. This response curve is valid for the inside detector sheets of the CR-39 stacks, whereas the outside surfaces yield a similar response as measured by Beth et ai.(1985). The sensitivity of the CR-39 used on Spacelab-1 is much less than known till now, but the resolution is better as the response curve is steeper at lower vt/v b values. We attribute this effect to the treatment of the stacks before installation. 60-

_~

N=365

r

q

10' 300

x x

250

x

•1 0 ~ .

o o

200

o~

i

x

150 1,0 t .

"tOO

x

50

+

x X

500

-100{)

1500 2000 2500 RESIDUAL RANGE [MICRONS]

3000

"lO=

10 3

REL2 00

~

"tO'

MeV. cm 2/g3

Fig.2 Conelength-range diagram of 101 tracks of the Fig.3 Response curves of CR-39. vt: 410 MeV/nuc Fe-56 beam. L-8 cones per trajectrack etch rate; v. : bulk e~ch o tory were measured. Cones of the same trajecrate; X: data from Henshaw et al.(1981),(6.25n-NaOH,70°C); tory are marked with the same symbol. The Y: Berkeley Fe-56 tracks and +: solid lines are calculated for the main tracks attributed to Ca-40 cosisotopes of the elements Z = 10 - 26 using mic rays, both this work(6n-NaOH, the response curve of Fig.3. 70°C); o: data from Beth et al. (1985),(6n-NaOH,70°C). From track measurements we derived a bulk etch rate v b = (1.01 etching conditions, and a response function vt/v b = I+O.0113"REL3"957±4.3%

(units of REL200~V:

IO3.MeV.cm2/g;

0.02) m/h at our 2.3
C~-39 USED FOR COSMIC RAY MEASUREMENTS

The tracks of the 39 particles classed as candidates to be "magnetically forbidden" were measured and thus nuclear charge and energy of the particles were determined as well as dip and azimuth angles were converted into zenith and azimuth angles relative to the earth's surface at the incident point. These angles have an accuracy of about ±5 degrees due to the , Earfh radius shuttle's motion and about ±I min uncertainty at determination of the impact time. Fig. h Calculated particle trajecWith the trajectory tracing method using the tory of an ion with a rigidgeomagnetic field published by Barraclough ity of 4.03 OV in the geomaget 8/.(i975) and on the assumption that we netic field (vertical incimeasured a particle of the main isotope, the dent at 50ON,60OE and 250 km trajectories of the 39 candidates were computed. altitude). We calculated with those incident points on the measured orbit intervals which had the minimum cutoff rigidity in that interval. As an example OEOOR. for these calculations a trajectory of a charged LATITUDE particle in the earth's magnetic field is plotted in Fig.4. 17 of the 39 candidates were found to be "magnetically allowe~' as vertical cutoff-values are GEOGR. not the same as cutoff values for other incident t ~ ...... LONGITUDE angles. 22 of the particles are "magnetically forbidden". ,oo ~o" - .... jpo _ ~-Possible explanations for this phenomenon are: - 20" I) There are irregularities in earth's magnetic field which were not taken into account in the magnetic field model. 2) The particles were not fully stripped as Fig.5 Incident points of 22 "geoexpected for cosmic ray particles and theremagnetical forbidden" particfore they had an increased rigidity. les projected on the earth. 3) The trajectories of the particles may have Dashed lines indicate: striped the atmosphere, which may cause a) maximum latitudes for energy loss and/or nuclear collisions before Spacelab-1 orbit, stopping in the stack. b) dipolar magnetic equator. A hint that the magnetotail of the field might SAA: South atlantic anomaly. be involved in letting the particles enter the magnetosphere up to "magnetically forbidden" places is given in Fig.6 where the local times (±0.5 hours) of the incident points at the impact time of the 22 forbidden particles are shown. 19 particles hit the stack at night times, three particles during day times. A significant 18 2 0 22 2'1. :~ accumulation on specific days was Loca! t i m e not detected. Fig.6 Local impact times of 22 "geomagnetlcally The optical horizon of Spacelab in forbidden" particles. 250 km altitude is at a zenith angle of 105 degrees. Trajectory tracing calculations show that particles might fall in with zenith angles under the optical horizon (Fischer, 1983). For six of the 22 particles we measured zenith angles between 113 and 126 degrees. Those particles must have approached quite near to the earth so that their trajectories might have striped the atmosphere. ~ig.6 shows a distribution of the incident points of the 22 "geomagnetically forbidden" particles projected on the earth. 19 particles hit the stack in southern latitudes, only 3 particles hit the stack in northern latitudes though the exposure time of the stack was equal in both hemispheres. Further studies on this phenomenon are in progress. Axis

,nf all trajectory

I : ;L

421

422

J. KRAUSE et al.

ACKNOWLEDGEMENT The authors like to thank the staff of the Berkeley accelerator for the ex~ experiment to the Fe-56 beam of Bevalac, Professor Wibberenz for his support federal government (BMFT) for subsidy under contract 01 QV 292. REFERENCES Barraclough, D.R., J.M.Harwood, B.R.Leaton and S.R.C.Malin (1975) Geophys. J. R. astr. Soc.(1975) 43, 645 - 659 Beth, M., W. Enge and G. Sermund (1985) Some new aspects on etching of CR-39 plastic detector, this conference, Roma Fischer, E.(1983) Berechnung yon Bahnen hochenergetischer lonen im Erdmagnetfeld, Diplomarbeit(DiplomalThesis), University Kiel, Germany Henshaw, D.L., S.Amin, V.M. Clapham, P.H.Fowler, D.J.Webster, A. Thompson and D.0'Ssullivan (1981) (Proc.11.Conf.,SSNTD,Bristol) Suppl. No.3 Nuc. Tracks, 115 - 118 Oschlies, K., R.Beaujean and W. Enge (1985) Measurement of low energy cosmic rays aboard Spacelab-1, this conference, Roma