Radiation Measurements 34 (2001) 277–280www.elsevier.com/locate/radmeas

Investigation of cosmic rays and their secondariesat aircraft altitudes

D. O’Sullivan ∗, D. Zhou, E. FloodDublin Institute for Advanced Studies (DIAS), Astrophysics Section, 5 Merrion Square, Dublin 2, Ireland

Received 28 August 2000; received in revised form 15 January 2001; accepted 8 March 2001

Abstract

A very extensive study of the radiation 1eld at aircraft altitudes has been carried out over the last few years. Theseinvestigations formed part of a European wide collaboration involving several laboratories with extensive experience in cosmicray research and=or dosimetry. Among the main topics investigated were the charge spectra, LET spectra, anisotropy and dosevalues. The measurements were performed on subsonic and supersonic 5ights covering a wide range of altitudes and latitudes.Several active and passive instruments were employed in these studies and some results obtained with nuclear track detectorsare described here. Comparisons are made with the results of other experiments and theoretical estimates using computercodes. c© 2001 Published by Elsevier Science Ltd.

Keywords: Galactic cosmic rays; Solar particles; Aircrew; Dosimetry at aircraft altitudes

1. Introduction

Cosmic rays are believed to originate from several possi-ble sources and recent research suggests that the bulk orig-inate from the gas and dust of the interstellar medium andare accelerated by strong shock waves driven by supernovaexplosions. Cosmic ray particles are made up of ∼ 98:5%hydrogen and helium and only 1.5% have charges greaterthan 2. Their average energy is about 1 GeV nucleon−1 andthey lose energy through ionisation interactions and nuclearinteractions with atoms of air as they penetrate deeply intothe Earth’s atmosphere. A very complicated radiation 1elddevelops as particles are generated by successive interac-tions of primary and secondary nuclei and a cascade ofhadrons is produced in the atmosphere. The intensity of par-ticles reaches a maximum at about 20 km above sea level(∼ 60 g cm−2). The relative abundances of di?erent parti-cles change with depth within the atmosphere and mainlymuons, which are decay products of charged mesons, reach

∗ Corresponding author. Fax: 353-1-6621477E-mail address: dos@cp.dias.ie (D. O’Sullivan).

sea level because of their weak interaction. The radiation1eld produced and consequently its e?ect on aircrew andfrequent travellers is a matter of some concern. Here weoutline the results of investigations carried out with solidstate nuclear track detectors to determine the characteristicsof this radiation 1eld and assess the impact on aircrew.

The risks associated with cosmic rays that penetrate intothe Earth’s atmosphere were highlighted by the recommen-dations of the International Commission of RadiologicalProtection (ICRP) in Publication 60 issued in 1991 whichincluded for the 1rst time exposure to cosmic radiation asoccupational exposure (ICRP 60). These coincided with asigni1cant increase in air travel and a trend towards highercruising altitudes for subsonic and supersonic commercialaircraft throughout the world. There was also concern thatthe relative biological e?ectiveness of the neutron compo-nent was being underestimated. Consequently, in the early1990s, the European Commission initiated a programme ofresearch which involved scientists in the 1elds of cosmicradiation, dosimetry and neutron physics to investigate thevery complicated radiation 1elds at aircraft altitudes.

Details of the extensive investigations undertaken duringsolar minimum are described in DIAS Report 99-9-1 (1999).

1350-4487/01/$ - see front matter c© 2001 Published by Elsevier Science Ltd.PII: S1350 -4487(01)00167 -6

278 D. O’Sullivan et al. / Radiation Measurements 34 (2001) 277–280

This paper describes a number of studies which formedpart of the overall programme, using solid state nucleartrack detectors. Measurements of the isotropy of particles ofcosmic ray origin, charge spectra and dose equivalent val-ues at subsonic and supersonic 5ight altitudes are reported.A comprehensive description of experimental and analysistechniques used are provided in O’Sullivan et al. (1999).

2. Isotropy of cosmic ray �ux

Outside the Earth’s atmosphere, galactic cosmic radiationis highly isotropic. As cosmic rays penetrate the Earth’s at-mosphere the resulting interactions cause signi1cant changesin the directions of particle 5ow but most models of par-ticle transport assume an isotropic distribution at depths of100–300 gm cm−2 (Heinrich et al., 1999) where most com-mercial aircraft operate at cruising altitudes. (Indeed earlyinvestigations showed that this was the case near the topof the atmosphere.) To investigate this hypothesis, a smallplatform was constructed with seven bases which steppedin 15

◦increments from 0

◦to 90

◦and provided 7 di?erent

elevations. The platform was 5own on a subsonic route be-tween Dublin and New York and, on a separate experiment,on a supersonic route between London and New York, forseveral months in each case. Following recovery, the detec-tors were processed and analysed as described in O’Sullivanet al. (1999).

Figs. 1a and b show the results for measurements at threedirections, namely, 0

◦; 45

◦and 90

◦. The integral 5uence

values for subsonic 5ights (mean cruising altitude ∼ 12 km)were 249:5± 19:65 (0

◦); 241:8± 20:9 (45

◦) and 238:44±

21:3 (90◦) particles cm−2 sr−1, demonstrating a very high

degree of isotropy. (Total 5ight exposure was not recordedfor this data.) Likewise, the data from supersonic 5ights(mean cruising altitude ∼ 17 km) indicate a similar highisotropy: 1:93 ± 0:18 (0

◦); 1:90 ± 0:17 (45

◦) and 1:80 ±

0:17 (90◦) particles cm−2 sr−1 h−1. Thus, the assumption

of isotropy used in these investigations is justi1ed.

3. Relative abundances of nuclei with Z¿ 2 at aircraftaltitudes

Small stacks of USF-4 CR-39 detectors were 5own onsubsonic and supersonic aircraft for periods of up to 2000 hto investigate the relative abundances of primary and sec-ondary nuclei with Z¿ 2. Following recovery, the detec-tors were processed and the individual tracks of 121 nucleipenetrating more than one plate of the stacks were carefullymeasured to determine etch rates and ranges for the gradientmethod of analysis (Fowler et al., 1977). Some of the heav-ier particles registered in each of the 20 plates of a stack,allowing very accurate measurement of the etch rates andgradients.

Fig. 1. (a) Integral LET spectra (Fluence) for cosmic rays at sub-sonic altitudes for 0

◦; 45

◦and 90

◦directions. (b) Integral LET

spectra (5ux) for cosmic rays at supersonic altitudes for 0◦; 45

◦

and 90◦

directions.

Fig. 2. Di?erential 5ux of cosmic ray nuclei at aviation altitudes.

Fig. 2 shows the result of these abundance investiga-tions, for all observed nuclei. It has been shown (Zhouet al., 1999) that the di?erential 5uence of elements in the

D. O’Sullivan et al. / Radiation Measurements 34 (2001) 277–280 279

region 26 Z6 8 observed at supersonic cruising altitudesof 17 km agree very well with the predictions of the SiegenUniversity HITCODE programme which calculates particle5uxes at di?erent depths in the Earth’s atmosphere, includ-ing the e?ects of solar modulation.

The extended spectrum for supersonic 5ights as shownin Fig. 2 which includes nuclei observed for some Z valuesup to Fe (Z = 26) can also be compared with that predictedby the HITCODE programme, although the sample is small.The total number of nuclei observed with Z¿ 8 is 9 incomparison to the predicted number 10.4, showing againremarkably good agreement.

4. Aviation route doses

During the period of solar minimum (1993–1998) a verycomprehensive investigation of aviation route doses was un-dertaken with the support of the European Commission, us-ing active and passive detectors. The investigation involved15 di?erent international air routes, including both subsonicand supersonic (O’Sullivan and Zhou, 1999). The value ofthese studies was greatly enhanced due to the availability ofseveral accelerator facilities where detailed calibration andintercomparison of detectors were carried out. The cosmicray reference 1eld at CERN played a major role in this re-gard, allowing investigators to calibrate and intercomparepassive and active detectors and to study detector response ina radiation 1eld of well determined characteristics (Birattariet al., 1997). Furthermore, these procedures helped to im-prove the reliability of results obtained on 5ights where itwas not possible to have all detectors on board at the sametime.

The passive nature of the detectors used in the studiesoutlined here make them ideal for long exposures on aircraftsince they do not require special operating conditions and donot interfere with aircraft navigation. Details of the process-ing and analysis methods employed are described in detailin O’Sullivan et al. (1999). High LET (¿ 5 keV �m−1)spectra were measured by investigating the short range re-coils produced by neutrons, and to a lesser extent protons,at aircraft altitudes. The accuracy of the methods used weresupported by the 1nding that the integral 5uence rates ofhigh LET events scaled with neutron 5uence rates calculatedwith the FLUKA code for the routes in question (O’Sullivanet al., 1999). Fig. 3a shows the integral LET Spectra (doseequivalent) for four subsonic routes during solar minimum(ICRP 60). The dose equivalent values (�Sv h−1) were3:56 ± 0:13 (Dublin–New York), 2:99 ± 0:29 (Milan–LosAngeles), 3:13 ± 0:52 (Milan–Tokyo) and 1:19 ± 0:12(Rome–Rio de Janeiro). Fig. 3b shows similar data fora supersonic route from London to New York (meancruising altitude, ∼ 17 km) for the three di?erent pe-riods listed. The values measured, (�Sv h−1), namely,7:24 ± 0:29; 7:13 ± 0:56 and 6:99 ± 0:64 indicate little orno variation of dose rates for this route during the period

Fig. 3. (a) Integral LET spectra (dose equivalent) of cosmic raysat subsonic altitudes. (b) Integral LET spectra (dose equivalent)of cosmic rays at supersonic altitudes, spanning a 31 month periodat solar minimum.

1994–1997. These values also agree very well with thosemeasured by the ANPA (Italy) and NRPB (UK) groups(Bartlett et al., 2000).

5. Comparison of measured values with the LUIN code

The LUIN computer code, developed by Keran O’Brien(O Brien et al., 1996) is one of the widely used programmesfor the simulation of cosmic ray propagation in the Earth’satmosphere. The programme provides di?erential and inte-gral 5ux estimates of cosmic rays and their secondaries aswell as the associated dose equivalents for di?erent 5ightaltitudes and latitudes. Table 1 shows the results for theMilan–Los Angeles and Rome Rio–Janeiro routes whereboth the neutron contribution and non-neutron contribu-tion (measured with thermoluminescent detectors of ANPA(Tommasino, 1999)) were measured for ICRP 60.

The agreement observed is very satisfactory and indi-cates that with further extensive investigation of route doses

280 D. O’Sullivan et al. / Radiation Measurements 34 (2001) 277–280

Table 1Comparison of measured dose rates with predictions of LUIN

Neutron dose Total dose LUIN totalrates rates dose rates(�Sv h−1) (�Sv h−1) (�Sv h−1)

Milan–Los Angeles 2:99 ± 0:29 4:8 ± 0:6 5.4Rome–Rio-Janeiro 1:19 ± 0:12 2:4 ± 0:3 2.0

globally, along with re1nement of codes such as LUIN, rou-tine estimates for aircrew exposure can be achieved on aroutine basis in the near future.

Acknowledgements

The authors wish to thank Aer Lingus, British Airwaysand Alitalia for provision of exposure facilities both on sub-sonic and supersonic aircraft. We also thank the sta? atthe GSI Darmstadt, and CERN centres, particularly DieterSchardt and Marco Silari respectively. Finally we thank theCommission of European Communities, Directorate GeneralXII for funding this project (Contract No: F14PCT950011).

References

Bartlett, D.T., Grillmaier, R., Heinrich, W., Lindborg, L.,O’Sullivan, D., Schraube, H., Silari, M., Tommasino, L., 2000.The cosmic radiation exposure of aircraft crew. Radiation

Research Congress Proceedings, Eleventh InternationalCongress, 18–23 July, 1999, pp. 719–723.

Birattari, C., Ferrari, A., HNofert, M., Otto, T., Rancati, T., Silari, M.,1997. Recent results of the CERN-CEC high energy reference1eld facility. CERN Divisional Report CERN=TIS-RP=97-12CF.

Dublin Institute for Advanced Studies (DIAS) Report 99-9-1, 1999.Study of Radiation Field and Dosimetry at Aviation Altitudes.www:cp:dias:ie=dias=cosmic=general=Publications=1999=Tech−Report=DIAS − 99 − 9 − 1=.

Fowler, O., Alexandre, C., Clapham, V., Henshaw, D., O’Ceallaigh,C., O’Sullivan, D., Thompson, A., 1977. High resolution studyof nucleonic cosmic rays with Z¿ 34. Nucl. Instrum. Methods147, 195–199.

Heinrich, W., Roesler, S., Schraube, H., 1999. Physics of cosmicradiation 1elds. Radiat. Prot. Dosim. 86, 253–258.

ICRP . 60, 1991. 1990 Recommendations of the InternationalCommission on Radiological Protection. Publication 60, Ann.ICRP 21 (1–3). Pergamon, Oxford.

O’Brien, K., et al., 1996. Atmospheric cosmic rays and solarenergetic particles at aircraft altitudes. Environ. Int. 22 (Suppl),S9–S44.

O’Sullivan, D., Zhou, D., 1999. Overview and present status ofEC research programme. Radiat. Prot. Dosim. 86, 4279–4283.

O’Sullivan, D., Zhou, D., Heinrich, W., Roesler, S., Donnelly, J.,Keegan, R., Flood, E., Tommasino, L., 1999. Cosmic rays anddosimetry at aviation altitudes. Radiat. Meas. 31, 579–584.

Tommasino, L., 1999. In5ight measurements of radiation 1elds anddoses. Radiat. Prot. Dosim. 86 (4), 297–301.

Zhou, D., Heinrich, W., O’Sullivan, D., Donnelly, J., Byrne, J.,Flood, E., 1999. Cosmic ray abundance at aircraft altitudes in theEarth’s atmosphere. Proceedings of the 26th ICRC, Salt LakeCity, USA, OG 1.1.27.