radiation measured with different dosimeters during sts-121 space mission

Acta Astronautica 64 (2009) 437–447www.elsevier.com/locate/actaastro

Radiationmeasuredwith different dosimeters during STS-121spacemission

D. Zhoua,b,∗, E. Semonesa, R. Gazaa,b, S. Johnsona, N. Zappa, M. Weylanda,R. Rutledgea, T. Lina

aJohnson Space Center-NASA, 2101 Nasa Parkway, Houston, TX 77058, USAbUniversities Space Research Association, 3600 Bay Area Boulevard, Houston, TX 77058, USA

Received 30 May 2008; received in revised form 10 August 2008; accepted 3 October 2008Available online 26 November 2008

Abstract

Radiation impact to astronauts depends on the particles’ linear energy transfer (LET) and is dominated by high LET radiation.Radiation risk experienced by astronauts can be determined with the radiation LET spectrum measured and the risk responsefunction obtained from radiobiology. Systematical measurement of the space radiation is an important part for the research onthe impact of radiation to astronauts and to make the radiation ALARA (as low as reasonably achievable). For NASA spacemissions at low Earth orbit (LEO), the active dosimeter used for all LET is the tissue equivalent proportional counter (TEPC)and the passive dosimeters used for the astronauts and for the monitored areas are the combination of CR-39 plastic nuclear trackdetectors (PNTDs) for high LET and thermoluminescence dosimeters (TLDs) and optically stimulated luminescence dosimeter(OSLDs) for low LET. TEPC, CR-39 PNTDs and TLDs/OSLDs were used to measure the radiation during STS-121 spacemission. LET spectra and radiation quantities were obtained with active and passive dosimeters. This paper will introduce thephysical principles for TEPC and CR-39 detectors, the LET spectrum method for radiation measurement using CR-39 detectorsand TEPC, and will present and compare the radiation LET spectra and quantities measured with TEPC, CR-39 PNTDs andTLDs/OSLDs.Published by Elsevier Ltd.

Keywords: Cosmic rays; Space radiation; Passive and active dosimeters; LET spectra

1. Introduction

The radiation field of particles in low Earth orbit(LEO) is very complicated and composed mainly ofgalactic cosmic rays (GCR), solar energetic particles,electrons and protons trapped in the south Atlanticanomaly (SAA) region of the Earth’s radiation belts,

∗Corresponding author at: Johnson Space Center-NASA, 2101Nasa Parkway, Houston, TX 77058, USA.

E-mail address: [email protected] (D. Zhou).

0094-5765/$ - see front matter Published by Elsevier Ltd.doi:10.1016/j.actaastro.2008.10.001

and albedo neutrons and protons from the Earth’s atmo-sphere. GCR consists of 98% protons and heavy ionsand 2% electrons and positrons.

Research indicates that radiation risk is dominated bythe radiation particles with high linear energy transfer(LET; � 10keV/�m water) and can be obtained basedon the risk cross section as a function of LET obtainedfrom radiobiology research and the LET spectrummeasured with radiation dosimeters. LET, especiallyhigh LET is the most important information neededto determine the radiation risk for astronauts. For the

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438 D. Zhou et al. / Acta Astronautica 64 (2009) 437–447

measurement of radiation LET spectrum, NASA-JSC(Johnson Space Center) uses active dosimeter, tissueequivalent proportional counter (TEPC), which is sen-sitive to LET from 0.2 to 1250keV/�m water and pas-sive dosimeter, CR-39 plastic nuclear track detectors(PNTDs), which is sensitive to high LET ( � 5keV/�mwater).

TEPC is sensitive to all kinds of charged particlesincluding GCR, trapped particles in SAA and solar en-ergetic particles, TEPC is also sensitive to neutrons.Radiation measured with TEPC can reflect clearly thecharacteristics of the radiation environment in LEO.There are large peaks for the dose rate measured withTEPC from the passage of space orbiter through theSAA. Therefore, data collected while the spacecraft waspassing through the SAA can be separated from datacollected outside the SAA, and in this way, the LETspectra from trapped particles can be separated from theLET spectra for GCR. However, due to the big volumeof the TEPC instrument and power supply, the NASA-JSC TEPC cannot be used as personal dosimeter forastronauts at present stage.

On the other hand, high LET radiation contributed byall kinds of charged particles (primary and secondary-recoils and fragments) including high charge (Z � 2)and high energy particles (HZEs) can be measured withCR-39 detectors. Neutrons can also be measured withCR-39 PNTDs through the charged secondaries pro-duced in the nuclear interactions between neutrons andthe CR-39 material, the spacecraft shielding, as well asthe internal structures and instruments.

CR-39 PNTDs can be used as personal dosimeters forastronauts and are so far the only personal dosimeterswhich can measure LET spectrum. The radiation riskfrom space radiation exposure dominated by high LETcan then be calculated using the LET spectrum mea-sured with CR-39 PNTDs and the risk cross section asa function of LET determined by radiobiology.

To compare the radiation quantities measured withTEPC and CR-39 PNTDs, a stack of CR-39 detectorswas attached to the cylinder wall of TEPC. Results in-dicate that radiation for high LET measured with TEPCand CR-39 PNTDs for STS-121 mission are in goodagreement.

In addition to the TEPC and CR-39 PNTDs, thermo-luminescence dosimeters (TLDs) and optically stimu-lated luminescence dosimeters (OSLDs) were also usedfor STS-121 space mission. Research indicates that theefficiency of TLDs and OSLDs is ∼100% for low LET(< ∼10keV/�m water) and the efficiency of CR-39PNTDs is ∼100% for high LET (> ∼10keV/�m wa-ter). Therefore, the radiation quantities for all LET can

be obtained by the combination of the results measuredwith TLDs/OSLDs and CR-39 PNTDs and the bestcombination point is at 10keV/�m water.

The LET spectra (differential and integral fluence,dose and dose equivalent) and radiation quantities insidethe spacecraft were measured by JSC-SRAG (SpaceRadiation Analysis Group) with TEPC, CR-39 PNTDsand TLDs/OSLDs for the STS-121 space mission (4–17July 2006, inclination 51.6◦, 307h).

This paper describes the physical principles forthe different radiation dosimeters, introduces the LETspectrum method using CR-39 PNTDs, presents andcompares the radiation results with high LET mea-sured by the TEPC and CR-39 detectors and the resultscombined from those measured with TLDs and CR-39PNTDs.

2. Physical principles for different radiationdosimeters

CR-39 PNTDs and TEPC deal with different physicalquantities: the CR-39 detector measures LET, while theTEPC measures lineal energy.

The linear stopping power S, contributed by atomiccollision of charged particles, is related to LET anddefined as Se = (dE/dx)elec, where ‘elec’ specifies elec-tronic (atomic collision). The LET or the restricted en-ergy loss is defined as L� =dE�/dx , where dE� is theenergy loss by a charged particle due to electronic col-lision, minus the sum of the kinetic energies of all theelectrons released with kinetic energies in excess of �.Research indicates that for CR-39, the electrons withenergies greater than � = 200ev are not the contribu-tors to the track formation. The restricted energy loss isoften written as LET� and the unrestricted energy lossas LET ∞ .

Lineal energy y is a microdosimetric quantity mea-sured by detectors such as the TEPC and has the sameunits as Se and LET. Lineal energy represents theamount of energy transferred to a microscopic volumeof material (e.g., an individual biological cell) and de-fined as y= �/x, where � is the energy transferred to amicroscopic volume of material from a charged particleand x is the mean chord length through the microscopicvolume.

A TEPC is an active detector filled with a gas; whencharged particles pass through the gas they loss energyand produce ionization. TEPC is operated in the pro-portional mode such that the signal generated is propor-tional to the initial ionization generated in the gas bythe incident charged particle. The tissues equivalenceof a TEPC is based on the composition of the wall that

D. Zhou et al. / Acta Astronautica 64 (2009) 437–447 439

defines the gas cavity as well as the composition of thegas itself.

TEPC is designed to measure lineal energy depositedin the sensitive microdosimetric volume which can sim-ulate the response of human tissue to radiation. Dataare recorded on an event-by-event basis such that onecan obtain a distribution of biologically relevant energydeposition events. Its tissue equivalence and large dy-namic range make it sensitive to photons, neutrons andcharged particles from electrons and protons to heavyions. TEPC can provide a reliable estimation of dosefrom protons through high atomic number, high energy(HZE) particles as well as photons and neutrons. Al-though y and LET are fundamentally different, the datain terms of y are not a direct replicate of the distribu-tions of fluence or absorbed dose as a function of LET,the average values of y, in particular dose-averaged yfor a mixed radiation field with a large number of par-ticles and broad range of energies and charges, are nu-merically similar to LET. Therefore, the difference forthe experimental results measured with TEPC and de-termined with CR-39 PNTDs is small and the resultsare comparable with each other.

The y response of TEPC to radiation can be obtainedby exposing TEPC to radiation sources (�-rays, elec-trons and ions). A detector response function is used toconvert lineal energy spectrum to LET spectrum.

NASA-JSC TEPC [1] consists of a circular cylinder,5.08cm long and 5.08cm in diameter, made of 1.9mmthick tissue equivalent plastic and filled with lowpressure propane gas. The detector simulates a 1�mdiameter biological cell and connected to a 256 ana-log to digital converter and is sensitive to the ion-izing particles of 0.2–1250keV/�m. The resolutionbelow 20keV/�m is in 0.1keV/�m steps and above20keV/�m is 5keV/�m steps. The lineal energy spec-trum is recorded every minute while the absorbed doseis calculated every 2 or 20s depending on the doserate. The lineal energy spectrum is converted to LETspectrum using the JSC-TEPC conversion function.

Comparing to other passive dosimeters, with thechemical composition C12H18O7, CR-39 is more simi-lar to the human tissue. Therefore, CR-39 detectors aremost suitable to simulate and represent the response ofhuman tissue to the radiation with high LET. The LETthreshold of the CR-39 PNTDs is about 5keV/�m wa-ter, enabling protons of energy up to about 10MeV aswell as HZE particles to be detected directly. Secondarycharged particles from nuclear interactions of higherenergy protons and of neutrons, in the detector materialitself or surrounding material, are also detectable. By ameasurement of the distribution of dose as a function

of LET, dose equivalent can be determined for energydeposition by all particles with LET > ∼5keV/�mwater.

The material of CR-39 PNTDs used by JSC group ismanufactured by American Technical Plastics Inc. Todecrease the radiation influence from natural radon �particles and to protect the surface of CR-39 material,a plastic film with a thickness of ∼60�m is covered onthe surface of CR-39 when manufacturing. The plasticfilm is removed when preparing CR-39 detectors forspace exposures or for accelerator exposures. The platesfor measuring background radiation are stored in spacecenters JSC and KSC (Kennedy Space Center).

When charged particles pass through CR-39 detector,they break the molecular bonds of the CR-39 polymerto form highly chemically reactive paths along their tra-jectories. These paths can be revealed as etched coneson the surfaces of the CR-39 detectors by chemical etch-ing for the CR-39 plates. The LET spectrum and theradiation quantities can then be determined with LETspectrum method using CR-39 detectors based on theLET calibration for CR-39 PNTDs.

The discussions for LET and lineal energy may foundin Benton [2] and Zhou et al. [3–5].

TLDs and OSLDs are made of solid inorganic crys-tal (see Table 1 for details) and measure the absorbeddose of ionizing radiation. TLDs and OSLDs accumu-late signal over the course of exposure and this signalis measured during the readout of the TLD and OSLD.When a charged particle passes through the crystal, theparticle loses energy by ionizing the atoms of the crys-tal. Kinetic energy from the charged particle is trans-ferred to electrons in the valence band, elevating theminto the conduction band. The electrons elevated intothe conduction band readily return to the lower energystate of the valence band and emit photons of visiblelight. TLDs and OSLDs contain impurities that can trapthe electrons before they reach the valence band. Whenheated (for TLDs) or optically stimulated (for OSLDs),the trapped electrons are released and give off photonsof visible light which can be amplified and measuredwith a photomultiplier tube. The quantity of visible lightmeasured by the photomultiplier tube is proportional tothe radiation dose deposited by the charged particle inthe crystal. The dosimeter material can be calibrated byexposing TLDs and OSLDs to known doses of radia-tion, after which a mathematical function for TLD orOSLD light output and absorbed dose can be obtained.TLDs and OSLDs are the best passive detectors for lowLET radiation measurement.

Table 1 collects the information for the TL/OSLdosimeters used by JSC-SRAG researchers.


Table 1Information for TL/OSL dosimeters used by JSC-SRAG .

Dosimeter type Composition Supplier Readout method

TLD-100 LiF:Mg,Ti Harshaw (now Thermo Fisher Scientific) TLTLD-300 CaF2:Tm (same as above) TLOSLD-300 Al2O3:C Landauer Inc. OSL (for 300s)OSLD-3 Al2O3:C Landauer Inc. OSL (for 3s)

The best combination of passive dosimeters for allLET is then TLDs and/or OSLDs and CR-39. The totaldose and total dose equivalent can be obtained from thecombined results, which should be comparable to thatmeasured by TEPC for the same shielding.

3. LET spectrum method for radiationmeasurement using CR-39 PNTDs

The LET spectrum method and the procedures forthe radiation measurement using CR-39 detectors canbe described briefly as: determination the purposesof the radiation measurement; detectors preparation;exposures of CR-39 detectors to space radiation, toaccelerator-generated heavy ions and protons (LETcalibration) and to the ground radiation (backgroundmeasurement); detectors recovery; chemical etch forCR-39 detectors; events recognition and data acquisi-tion with optical microscope; data analysis/calculationto obtain differential and integral LET spectra (fluence,absorbed dose, and dose equivalent). The central partsof the method are data acquisition, LET calibration,and LET spectrum generating and will be introducedbriefly below.

3.1. Radiation exposure of CR-39 detectors in space

STS-114 and STS-121 space missions were con-ducted in 2005 (26 July–9 August, 333h) and 2006(4–17 July, 307h), respectively, with the same inclina-tion of 51.6◦. To investigate radiation with high LETusing both active dosimeter TEPC and passive CR-39dosimeters, CR-39 detectors were attached to the outerwall of the TEPC cylinder. Due to the limited availablespace near TEPC cylinder, TLDs and OSLDs were notused.

There were also seven stacks of passive radiationdosimeters (PRDs) composed of CR-39 PNTDs, TLDs,and OSLDs for the STS-121 flight. Six stacks of pas-sive dosimeters are used for six selected locations in-side the spacecraft and one stack was left on the groundas the control stack for the measurement of ground

Fig. 1. Cross section for the stack of JSC-SRAG passive dosimeters.

background radiation. The six selected locations are:one, left mid-deck, above external airlock hatch; two,right mid-deck, outer wall, starboard; three, left mid-deck, above ingress/egress hatch; four, flight deck (cen-ter ceiling), aft centerline observation window; five,right flight deck, panel above locker L-10; six, left flightdeck, panel above locker R-11.

Fig. 1 shows the cross section of the combined stackof passive detectors for the STS-121 mission. TLD100,TLD300, OSLD300s and OSLD3 were used. The col-umn mass density of TLDs and OSLDs is ∼0.8g/cm2.The thickness of the CR-39 plate is ∼600�m with acolumn mass density of ∼0.8g/cm2. A stack of PRDdosimeter consists of two CR-39 detectors and somematerial of TLDs and OSLDs.

After exposure and recovery, the CR-39 detectorsfrom STS-121 flight, along with the plates for groundbackground radiation, were etched chemically (NaOH,6.25N, 60 ◦C). To avoid underestimating the LET val-ues for events with very short ranges, a shorter timeetch was used.

3.2. Data acquisition and manual scan

Due to the disadvantages of automatic scan [6] andthe unique advantages of manual scan [3–5,7–12] fordata acquisition in radiation research using CR-39


detectors, in JSC-SRAG work all data used for CR-39detectors were obtained by manual scan.

An important advantage of the manual scan is to rec-ognize and collect data for HZE particles in the sameprocedure of the scan for all particles. The method tofind out HZEs can be described briefly as below. In theprocedure of data scan on the top surface of the CR-39detector, every selected etched cone is focused down-wards to the bottom surface along the direction of majoraxis for the top cone to determine the event is coinci-dent or not, if a symmetric bottom cone is found, theevent is coincident and the particle is selected as longrange HZE particle.

Both primary and secondary HZEs can be recordedwith CR-39 PNTDs. The percentage of secondary HZEswhich produced near the surface of CR-39 (within ∼adistance of bulk etch from the surface) by the interac-tion between the primary charged particles and CR-39material can be obtained by experiment. To determinethe percentage of the secondary HZEs, CR-39 detec-tors exposed to protons (1GeV, BNL, 2004) and irons(1GeV/n, BNL, 2006) were used and a huge numberof events were scanned. The results indicate that num-ber of secondary HZEs is< 1% of the number of beamparticles. Therefore, the HZE particles observed withCR-39 detectors using the approach described above aredominated by the primary GCR nuclei.

3.3. LET calibration for CR-39 detectors

The relationship between LET200 in CR-39 and theetch rate ratio used by JSC-SRAG was determined byexposing CR-39 with protons and heavy ions generatedby accelerators. The energies of the protons and ionsare dedicatedly selected so as to give a wide range ofLET response which can satisfy the requirements forthe research of space radiation. The calibration of CR-39 detectors was conducted by JSC-SRAG in 2004 and2005 at BNL, TAMU (Texas A and M University) Cy-clotron Institute, and HIMAC.

Fig. 2 shows the LET calibration used for space mis-sion STS-121. The description of the method and pro-cedure in detail for CR-39 LET calibration can be foundin Zhou et al. [11–14]. The present LET calibration ofSRAG is covered from ∼5 to ∼1000keV/�mwater, sat-isfies the requirements for the LET spectrum work.

3.4. LET spectrum generating

Following etch, the thickness and the mass before andafter etch for the CR-39 plates were measured and bulk

Fig. 2. LET calibration of CR-39 detectors.

etch B was calculated by the Henke’s formula [15]:

B = (m1 − m2)T22m2

(1 − pT2

2Ad

)

where m1 is the detector mass before etch; m2, the massafter etch; T2, the detector thickness after etch; p, thedetector perimeter; Ad, the detector surface area.

Events were recognized and selected with manualscan and the major and minor axes of the etched trackcones on the CR-39 surfaces were measured.

The etch rate ratio can be calculated by Somogyi’sformula [16]:

S =

√√√√1 + 4( a

2B

)2 / [1 −

(b

2B

)2]2

where a and b are the major and minor axes of the etchedcones, respectively. LET values can then be calculatedusing LET calibration for CR-39 detectors.

Research indicates that the arrival directions of theGCR are isotropic [17–19] and GCR is the main con-tributor for space radiation. Therefore, the space radia-tion in LEO is nearly distributed isotropically.


For a radiation field distributed isotropically the dif-ferential fluence is calculated by:

F = (2�A cos2 �cut)−1 dN

dLET

where F is the differential fluence in particles/(cm2.sr.keV/�m water); A, the scanned detector area; dN,the number of events; dLET, the LET bin and �cut, thecutoff dip angle above which the detection efficiency ofCR-39 detectors is 100% [12,20].

The net differential fluence for space exposure is ob-tained by subtracting the ground background radiationfrom the total differential fluence, which is contributedby both space radiation and background radiation.

If the radiation is distributed isotropically, the differ-ential absorbed dose (Gy) is then

Dose = 4� × 1.6 × 10−9 × LET∞ × F

where LET ∞ in keV/�m water is the LET value at thecenter of the relevant LET bin.

The differential dose equivalent is obtained asDose×Q, where Q is the quality factor ICRP-60.

The integral spectrum is generated by summing thedifferential spectrum from high LET to low LET. The

Fig. 3. (a) Differential LET spectra of flux measured with TEPC and CR-39 near TEPC. (b) Differential LET spectra of flux measured withTEPC and CR-39 at different locations.

average quality factor is then calculated by

Q(�LET) = integral dose equivalent (�LET)

/integral absorbed dose (�LET)

The relationship between LET ∞ water and LET200CR-39 can be expressed as [2]

log(LET∞water)=0.1689+0.984log(LET200CR−39)

The error of the radiation quantities stems from thecounting statistics for the events measured.

The details of the LET spectrum method and the pro-cedures using CR-39 PNTDs can be found in O’Sullivanet al. [8,20] and Zhou et al. [3–5,9–14].

4. Radiation measured with TEPC and CR-39PNTDs

4.1. Radiation measured with TEPC and determinedwith CR-39 detectors

Figs. 3a and b and 4a and b show the differential spec-tra and integral spectra of flux for STS-121 measuredwith TEPC and PRD CR-39 PNTDs as well as CR-39detectors near TEPC. In the figures, ‘trapped’ means the


Fig. 4. (a) Integral LET spectra of flux measured with TEPC and CR-39 near TEPC. (b) Integral LET spectra of flux measured with TEPCand CR-39 at different locations.

trapped particles in SAA. Figs. 3a and 4a show GCR isdominant and, in high LET ( � 10keV/�m water) re-gion the fluxes measured with TEPC (GCR+trapped)and CR-39 PNTDs near TEPC are in good agreement.Figs. 3b and 4b indicate that due to the different shield-ing for different locations inside the spacecraft the spec-tra of flux are different. Figs. 3b and 4b also include thedifferential and integral spectra of HZE particles col-lected from PRD3 CR-39 and CR-39 near TEPC.

Fig. 5 shows the integral LET spectra of dose equiv-alent (ICRP 60) measured with TEPC. The figure in-dicates that the contribution of dose equivalent fromGCR and SAA trapped particles for STS-121 is ∼79%and ∼21% of total dose equivalent, respectively, (seeTable 2 in Section 4.2 for detail). The figure also showsan excellent agreement between the dose equivalentwith high LET measured by TEPC and CR-39 PNTDs.

Fig. 6 shows the integral LET spectra of doseequivalent (ICRP 60) measured with CR-39 PRDs forSTS-121. In the figure, and in the following figuresand tables ‘all’ means all kinds of particles (primaryand secondary including HZE particles). In Fig. 6, thetop six curves are the integral LET spectra of dose

equivalent contributed by all kinds of particles forPRDs. The lowest curve shows the integral LET spec-trum of dose equivalent for HZE particles of PRD3.

Table 2 collected the results measured with theCR-39 detectors. The table shows that the dose anddose equivalent is high at location 2 and 3. The tablealso shows that quality factor ( � 10keV/�m water)determined for different areas is nearly the same. Thisimplies that the LET spectra profile for the differ-ent locations is similar, as showed in Fig. 3b and 4b.For PRD3, the contribution of the dose equivalent( � 10keV/�m water) from HZE particles is ∼31% ofthe total dose equivalent from all kinds of particles.

4.2. Results and comparisons of the radiation measuredwith TEPC and CR-39 for STS-114 and STS-121

Fig. 7 shows the integral LET spectra of dose equiv-alent (ICRP 60) measured with TEPC and CR-39 detec-tors attached to the wall of TEPC tubes for the STS-114and STS-121 flights. The top four curves of dose equiv-alent for all kinds of particles show good agreement intheir spectrum profiles. The lower two curves are for


Fig. 5. Integral LET spectra of dose equivalent measured with TEPCand CR-39 near TEPC.

Fig. 6. Integral LET spectra of dose equivalent measured with CR-39PNTDs.

Table 2Results measured with CR-39 PNTDs for different locations .

Dosimeter Absorbed dose Dose equivalent Quality factorlocation (mGy) (mSv)

PRD1 (total) 0.36 ± 0.04 4.26 ± 0.47 11.72 ± 1.31PRD2 (total) 0.49 ± 0.06 5.74 ± 0.68 11.70 ± 1.39PRD3 (total) 0.48 ± 0.04 5.59 ± 0.48 11.65 ± 1.00PRD3 (HZE) 0.11 ± 0.01 1.72 ± 0.17 16.17 ± 1.55PRD4 (total) 0.38 ± 0.04 4.41 ± 0.43 11.70 ± 1.13PRD5 (total) 0.47 ± 0.04 5.46 ± 0.47 11.74 ± 1.00PRD6 (total) 0.41 ± 0.05 4.73 ± 0.64 11.63 ± 1.56

STS-121, � 10 keV/�m water, ICRP 60.

Fig. 7. Integral LET spectra of dose equivalent measured with TEPCand CR-39 PNTDs.

HZE particles determined with the CR-39 detectors, theprofiles of the two spectra are also in good agreement.

Table 3 presents a comparison for the radiation quan-tities measured with TEPC for STS-114 and STS-121.The agreement is excellent between the radiation resultsmeasured for STS-114 and STS-121. Data in the tableindicate that the dominant contribution of dose equiv-alent is from GCR for STS space missions with an in-clination of 51.6◦ in LEO.

Table 4 presents a comparison for the radiation quan-tities with high LET ( � 10keV/�m water) measured


by TEPC and CR-39 PNTDs attached to TEPC for STS-114 and STS-121 flights. The comparison of dose anddose equivalent measured with TEPC and determinedwith CR-39 PNTDs shows excellent agreement for bothSTS 114 and STS-121 flights. In the table and in Fig. 8“CR-39 All” means all kinds of particles, primary andsecondary, including HZE particles.

The range of HZE particles is greater than the thick-ness of a plate of CR-39 and can be recognized andcollected as multi-coincidence events. GCR is the dom-inant contributor to HZE particles based on the relatedresearch results on the percentage of secondary HZEsin total beam particles discussed earlier. The experi-mental results determined by the CR-39 PNTDs nearTEPC indicate that the contribution of dose equivalent( � 10keV/�mwater) from HZE particles is about 26%and 27% of total dose equivalent for STS-114 and STS-121, respectively.

Fig. 8 shows the average radiation quality factors fordifferent dosimeters and for different observed parti-cles. The average quality factors ( � 10keV/�m water),representing the radiation effect of high LET are listedin Table 3. The table indicates that the average qualityfactors for long range HZE particles are much higher

Table 3Results measured with TEPC for STS-114 and STS-121 .

Dose rate(�Gy/d)

Dose equivalentrate (�Sv/d)

Qualityfactor

GCRSTS-114 94.50 302.28 3.20STS-121 93.08 318.83 3.43

TrappedSTS-114 46.98 93.24 1.98STS-121 42.12 83.23 1.98

CombinedSTS-114 141.48 395.52 2.80STS-121 135.20 402.06 2.97

ICRP 60, 0.2−1250 keV/�m water.

Table 4Results measured with TEPC and CR-39 PNTDs attached to TEPC .

Mission Flight time (hour) Detector Absorbed dose (mGy) Dose equivalent (mSv) Q factor

STS-121 (July 2006) 307 TEPC (all) 0.34 ± 0.01 3.83 ± 0.10 11.16 ± 0.29CR-39 (all) 0.34 ± 0.02 4.00 ± 0.28 11.77 ± 0.82CR-39 (HZE) 0.06 ± 0.01 1.07 ± 0.14 17.62 ± 2.33

STS-114 (July–August 2005) 333 TEPC (all) 0.35 ± 0.01 3.96 ± 0.10 11.17 ± 0.29CR-39 (all) 0.35 ± 0.02 4.14 ± 0.26 11.73 ± 0.74CR-39 (HZE) 0.06 ± 0.01 1.07 ± 0.10 16.51 ± 1.67

STS-114 and STS-121, ICRP 60, � 10 keV/�m water.

than those for short range particles. Long range HZEparticles cannot be effectively shielded and can makean important contribution to the biological impact andrisk for astronauts.

5. Results combined from those measured withTLDs/OSLDs and CR-39 PNTDs

As mentioned in the introduction, the radiation quan-tities for all LET can be obtained by the combination ofresults measured with TLDs/OSLDs and CR-39 PNTDsand the best combination point is at 10keV/�m water.The combination method and procedures in detail can

Fig. 8. Quality factor measured with TEPC and CR-39.


Table 5Results combined from that measured with TLDs/OSLDs and CR-39 PNTDs .

Dosimeterlocation

TLD/OSLDtype

TLD/OSLDtotal dose(mGy)

Dose: low LET( � 10keV/�m)Q = 1(mGy)

Dose: high LET( � 10keV/�m)Q > 11 (mGy)

Total dose:all LET(mGy)

Total doseequivalent(mSv)

Q factor

PRD1 TLD-100 2.39 ± 0.03 2.18 0.36 ± 0.04 2.54 ± 0.05 6.44 ± 0.47 2.53 ± 0.19TLD-300 2.38 ± 0.05 2.10 2.47 ± 0.06 6.37 ± 0.47 2.58 ± 0.19OSLD-300 2.29 ± 0.05 2.08 2.45 ± 0.06 6.35 ± 0.47 2.59 ± 0.19OSLD-3 2.49 ± 0.07 2.24 2.61 ± 0.08 6.50 ± 0.48 2.50 ± 0.18






PRDs of STS-121, ICRP 60, combined at LET = 10 keV/�m water.

be found in Zhou et al. [21]. The combined results forPRDs of STS-121 are collected in Table 5.

The combined results indicate that the contribution ofdose equivalent from high LET and low LET particles is∼68% and ∼32% of total dose equivalent, respectively,and the quality factors for all LET is from 2.53 to 2.68for radiation at different locations inside the spacecraft.

On the other hand, the value of quality factors mea-sured with TEPC for LET � 0.2keV/�m is ∼2.75 forSTS-121. Although TEPC was at a location differentfrom those for PRDs and its LET threshold is higherthan that for TLDs and OSLDs, theQ values from TEPCand from the combined passive detectors are in goodagreement.

Researches conducted for the influence of radiationon tissues and on astronauts [22–26] have shown thatthe radiation impact is strongly dependent on both thedose absorbed and the LET value of the radiation par-ticles, the higher the LET value, the more harmful theradiation is. The radiation risk on astronauts can beestimated from the researches in the areas of radiobi-ology and the radiation measurements for astronauts.Therefore, the LET spectrum for high LET particles

measured determined with CR-39 dosimeters in spacemay play a unique and important role for the future in-depth research on the space radiation.

6. Conclusions

LET spectra (differential and integral fluence, doseand dose equivalent) were measured with active TEPC(all LET) and passive dosimeter CR-39 PNTDs (highLET) and the radiation quantities for all LET werealso obtained by combining the results measured withTLDs/OSLDs (low LET) and CR-39 PNTDs (highLET) for the STS-121 space mission. The experimentalresults for high LET radiation measured with TEPCand CR-39 PNTDs show excellent agreement. The LETspectra for high LET radiation measured with CR-39personal dosimeters are important and useful for theresearch on the impact and risk of high LET radiationon astronauts. Research on high LET related to spaceradiation and radiobiology should be emphasized. JSCwill continue to use both active and passive dosime-ters for the investigation of space radiation in thefuture.


Acknowledgment

The authors wish to thank all those who assisted themin their work at NSRL, BNL, TAMU, HIMAC, STS-114, and STS-121 mission.

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