Singh N. (ed.) Radioisotopes - Applications in Physical Sciences

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History of Applications of Radioactive Sources in Analytical Instruments for Planetary Exploration

169

Table 3. Characteristics of the AXS instrument for MUSES-C mission.

Radioisotopes – Applications in Physical Sciences

170

The MUSES-C mission suffered several long delays and NASA eventually cancelled its

participation on this mission. The Japanese, however, went ahead with the mission that

was renamed Hayabusa after the launch in 2003 and succeeded in bringing back a small

amount of asteroid material from asteroid Itokawa in June, 2010. Despite the cancellation,

the AXS was designed, built and fully flight qualified. It is now available for some

potential future mission.

6. Summary

We have described here some of the techniques and analytical instruments that have been

developed in the past half century and used for space applications. Many other space

analytical instruments have used radioactive sources in one way or another that have not

been described here. Some of the developed techniques had a profound influence in

developing analytical instruments that are used today in many terrestrial laboratories

around the world.

Table 4 shows some of the radioisotopes used in different analytical instruments on space

missions. Besides the

242

Cm and

244

Cm that were used in the APXS, the Mössbauer

experiment on the MER mission used about 400 millicuries of

Co at the start of the

mission, to obtain the mineralogy of the iron bearing rocks [10]. Similarly,

Fe has been

used in an XRF instrument on Viking mission in 1976 [11] and on the Beagle 2 mission in

2003 [12].

Isotope

Half Live

Time

Energy,

Type

Space Mission

1. Cm-242 161 days 6.1 MeV, α ASI Surveyor Lunar Missions, 1967-1968

2. Cm-244 18.1 years 5.8 MeV, α

Phobos1@2(‘88), Mars96(‘96),

Pathfinder(’97),

MER(2004), MSL(2011), Rosetta (2002)

3. Fe-55 2.7 y 5.9 keV, x Viking 1&2 (1976), Beagle 2 (2003)

4. Co-57 271 days

14, 122 keV,

Viking 1&2 (1976), MER (2004)

5. Am-241 432 y

5.49

MeV,α,γ

XRF

6. Pu-238 87.74 y 5.5 MeV, α

Radioisotope Thermoelectric Generators

(RTG), Radioactive Heating Units (RHU),

many space missions

7. Pu-239 2.4x104 y 5.15 MeV,α Calibrations

Table 4. The most common radioisotopes used in the analytical instrument for space

applications

History of Applications of Radioactive Sources in Analytical Instruments for Planetary Exploration

171

7. References

[1] Turkevich, A., “Chemical analysis of surfaces by use of large-angle scattering of heavy

charged particles”, Science, 134, 672, 1961.

[2] A. L. Turkevich, W. A. Anderson, T. E. Economou, E. J. Franzgrote, H. E. Griffin, S. L.

Crotch, J. H. Patterson and K. P. Sowinski. "The Alpha-Scattering Chemical

Analysis Experiment on the Surveyor Lunar Missions”. Jet Propulsion Laboratory

Technical Report 52-1265, pp. 505-82, June 15, 1968.

[3] Patterson, J.H, A. L. Turkevich and E. J. Franzgrote, Chemical analysis of surfaces using

alpha particles, J. Geophys.Res., 70, 1311, 1965.

[4] Thanasis E. Economou, Anthony L. Turkevich, Keneth P. Sowinski, James H. Patterson

and Ernest J. Franzgrote, The Alpha-Scattering Technique of Chemical Analysis, J.

Geophys. Res., 75, No 32, 6514, 1970.

[5] Thanasis E. Economou,Anthony L. Turkevich and James H. Patterson, An Alpha Particle

Experiment for Chemical Analysis of the Martian Surface and Atmosphere, J.

Geophys. Res., 78, No 5, 781, 1973.

[6] Patterson, J.H, Ernest J. Franzgrote,A. L. Turkevich, W.A. Anderson, T.E. Economou,

H.E. Griffin, S.L. Groach and K.P. Sowinski, Alpha Scattering Experiment

on Surveyor 7: Comparison with Surveyor 5 and 6, J. Geophys. Res., 74, 6120,

1969.

[7] James H. Patterson, Harry E. Griffin, E. Philip Horwitz, and Carol A. Bloomquist,

Preparation of High Level Alpha-Particle Sources for the Surveyor Alpha

Scattering Experiment, Nuclear Technology, Vol. 18, 277-285, 1073.

[8] Thanasis E. Economou and Anthony L. Turkevich. "An Alpha Particle Instrument with

Alpha, Proton and X-ray Modes for Planetary Chemical Analyses". Nucl. Instr. &

Methods 134, 1976, p. 391-399.

[9] Hovestadt, D., et al., PHOBOS, Proceedings of the International Workshop, Moscow,

1986, p 302

[10] Economou, T.E, J.S. Iwanczyk and R. Rieder, A HgI

X-ray Instrument for the Soviet

Mars '94 Mission, Nucl. Instr. & Methods A322,633-638, 1992.

[11] Linkin, V., et al., “A Sophisticated Lander for Scientific Exploration of Mars :

Scientific Objective and Implementation”, Planetary Space Science Journal, Vol. 46,

Issues 6-7, 1998, pp 717-737.

[12] Rieder, R., H. Wänke, T. Economou and A. Turkevich; “Determination of the Chemical

Composition of Martian Soil and Rocks: The Alpha-Proton-X-Ray Spectrometer”, J.

Geophys. Res. 1997 pp. 4027-4044.

[13] Rieder, R., T. Economou, J. Brückner, G. Dreibus, H. Wänke and A. Turkevich, First

Elemental Analysis of Martian Surface by the Mobile Alpha Proton X-ray

Spectrometer Attached to Mars Pathfinder Rover Sojourner. Meteoretics and

Planetary Science, Vol. 32, No. 4, A107, 1997.

[14] Rieder, R., T. Economou, H. Wänke, A. Turkevich, J. Crisp, J. Brückner, G.

Dreibus,H.Y. McSween, Jr., The Chemical Composition of Martian Soil and Rocks

Returned by the Mobile Alpha Proton X-ray Spectrometer: Preliminary Results

from the X-ray Mode, Science Vol. 278, (1997)1771-1774.

Radioisotopes – Applications in Physical Sciences

172

[15] C. Nicole Foley, Thanasis E. Economou and Robert N. Clayton, Final Chemical Results

from the Mars Pathfinder Alpha Proton X-ray Spectrometer, J. Geoph. Res. Vol.

108, No 12, doi:10.1029/2003JE002019, 2003.

[16] Rieder, R., et al., J. Geophys. Res., 108(E12), doi:10.1029/2003JE 002150, 2003

[17] http://www.esa.int/esaMI/Rosetta

[18] Klingelhoefer, G., J. Geophys. Res., 108, p. 8067, 2003

[19] Clark,B.C., et al., J. Geophys. Res., 87, p.10059, 1982

[20] Beagle 2 mission to Mars in 2003, http://www.beagle2.com/index.htm

[21] V. Radchenko, B. Andreichikov, H. Wanke, V. Gavrilov, B. Korchuganov, R. Rieder, M

Ryabinin, and T. Economou, Curium-244 alpha-sources for space applications,

Applied Radiation and Isotopes, 2000 Oct., 53(4-5):821-824.

[22] Economou, T.E, The Chemical Composition of an Asteroid Surface by the Alpha X-Ray

Spectrometer on the MUSES-C Mission Lander, In Lunar and Planetary Science

XXXI, 2000, 1861.pdf

Part 2

Radioisotopes in Environment

Environmental Dosimetry –

Measurements and Calculations

Mats Isaksson

Department of Radiation Physics, Institute of Clinical Sciences, The Sahlgrenska Academy,

University of Gothenburg

Sweden

1. Introduction

According to UNSCEAR (2008) the largest contribution to external exposure comes from

naturally occurring, gamma-emitting, radioactive elements in the ground. Apart from

these elements are members of the decay chains starting with

238

U and

232

Th, respectively.

Both of these radionuclides have half-lives comparable to the age of the earth and have been

present in the earth’s crust since its formation. The half-life of

K is of the same order of

magnitude. However, nuclear weapons fallout and debris from accidents at nuclear facilities

may at some sites contribute even more to the exposure.

Calculation of the external dose rate from field gamma spectrometric measurements

requires knowledge of how the primary fluence rate, i.e. the number of unscattered photons

per unit area and time, from different distributions of radioactive elements, at or in the

ground, affects the detector response. Likewise, determination of external dose rate based

on gamma spectrometry of soil samples must include a calculation of the primary fluence

rate from the actual soil inventory.

The main topic of this chapter is to describe, and compare, some of the methods that can be

used to estimate external dose rate from environmental sources of ionizing radiation from

radioactive elements. The external dose rate from radioactive sources at or below ground

may be estimated in several ways. One method is to simply measure the dose rate with a

properly calibrated intensimeter or other type of dose meter (e.g. ionization chamber or

thermoluminescent dosimeter, TLD). Another method is to calculate the dose rate based on

gamma spectrometric measurements, e.g. field gamma spectrometry or soil sampling.

However, due to assumptions of the relation between primary and scattered radiation, as

well as calibration requirements, the dose rate estimations from these methods seldom agree

in practical situations. The topics mentioned above will be exemplified with reference to

actual measurements by field gamma spectrometry and intensimeter, as well as model

calculations of fluence rates from different environmental geometries.

2. Primary fluence rate from different source geometries

The quantity fluence has been defined by the International Commission on Radiation Units

and Measurements, ICRU, (ICRU, 1998) as

= dN/da, i.e. the number of particles per unit

area. According to the definition, dN is the number of particles (here photons) incident on

Radioisotopes – Applications in Physical Sciences

176

the area da, which should be the cross-sectional area of a sphere. This definition of the area

is chosen because the definition of fluence should be independent of the direction of the

radiation field. The fluence rate is defined in terms of the increment of the fluence, d

during the time interval dt as d

/dt.

Photons emitted from a radioactive source in a medium may undergo different kinds of

interactions. Some of the photons pass through the medium without interacting and these

will constitute the primary fluence at the detector position. Other photons, originally not

directed towards the detector, may undergo Compton scattering and hit the detector. These

latter photons, or scattered radiation, will also contribute to the detector signal. The relation

between primary and scattered radiation is given by the build-up factor, which depends on

several parameters (e.g. photon energy and material between source and detector) and must

be determined by experiments or Monte Carlo-simulations.

2.1 Volume source

Figure 1 shows the geometry used for calculating the fluence rate at a reference point

outside the active volume. In this case we consider a homogeneously distributed source in

the ground, as an example. The number of photons emitted per unit time from a volume

element dV at the depth z in the ground is given by S

(z,r,

)⋅dV, where dV equals

dr⋅dR⋅d

Fig. 1. The geometry used for calculating the primary fluence rate at the position of the

detector, from a volume element in the ground. The volume element is considered as a point

source.

The differential (primary) photon fluence rate at the detector position, taking into account

self-attenuation in soil (

) and attenuation in air (

) is given by

()

/cos /cos

Rh h

SdVe

−− −⋅

⋅



(1)

The contribution from the volume element dV is thus simply treated as the primary fluence

rate from a point source at the distance of R from the detector. The exponential function

Environmental Dosimetry – Measurements and Calculations

177

accounts for the attenuation in the materials between the source and the detector (soil and

air). The source strength, S

, is related to the activity, A, through the relation S

⋅dV = A⋅Σf

where f is the probability for emission of a photon (gamma-ray) of a certain energy in each

decay of the radionuclide.

The total primary photon fluence rate can now be determined by integrating the differential

fluence rate over the whole volume, i.e. summing the contributions from each volume element

dV. Changing to spherical coordinates will facilitate the integration and dV is then expressed

as dV = dr⋅dR⋅d

= R

⋅sin

⋅d

⋅dR. The primary photon fluence is thus given by

()

/cos /cos

/cos

sin

Rh h

SR e

ddRd

μθμθ

ηθ

∞

−− −⋅

⋅⋅ ⋅





(2)

The above expression can be calculated analytically for a few cases of distributions and these

will be exemplified by two kinds of homogeneous distributions: an infinite volume source

and an infinite slab source.

2.1.1 Infinite volume source

An infinite volume source can often be assumed when considering naturally occurring

radionuclides in the ground. These are the members of the uranium and thorium decay

chains, together with

K. Since the mean free path of photons from naturally occurring

radionuclides is about 10 cm (based on 3 MeV photons and a soil density of 1.5 g⋅cm

-3

) a soil

volume of this thickness may be considered as an infinite source.

Integrating Eq. 2, we find that

[]

ηπ



(3)

and

()

/cos

edRe e

μθ

μμ

∞

−⋅−

−⋅

⋅



=⋅−⋅ =







(4)

The expression for the primary fluence rate then reduces to

/cos

sin

μθ

θθ

−⋅

=⋅





(5)

This integral can be solved by variable substitution and by using the properties of the

exponential integrals, E

(x), defined as

()

Ex x dt

∞

−



(6)

The substitution t =

⋅h/cos

enables us to rewrite Eq. 5 as

Radioisotopes – Applications in Physical Sciences

178

()

/cos /

/cos

sin

cos

sin

cos

sin

a a

ss a

Se S

dedt

he dt hdt

μπ

μθ

μμ

φθθ

μμ μθ

μθ μ

μμ

μθ

⋅

−⋅

−

⋅

∞∞

−

⋅⋅

==⋅⋅=

⋅⋅ ⋅⋅

=⋅⋅ ⋅⋅ = ⋅

⋅⋅





(7)

By using Eq. 6 with x =

⋅h and n = 2 , the primary fluence rate can be written

()

φμ

=⋅

⋅



(8)

The exponential integral E

(x) equals 1 for x = 0 and decreases monotonically to 0 as x

approaches infinity; the value of E

(x) can be found in standard mathematical tables and

graphs. A special case occurs when the reference point (detector) is at the surface of an

infinite volume source, i.e. h = 0. Hence there is no attenuation in air and

⋅h = 0, giving

(0) = 1. Eq. 8 then reduces to

⋅



(9)

2.1.2 Infinite slab source

Ploughing after fallout of radionuclides on arable land may lead to a homogeneous

distribution of the radionuclides in the upper part of the soil, whereas the deeper soil may

be considered inactive with regard to the radionuclides in the fallout. The fluence rate from

such a slab source (of infinite lateral extension) of thickness z can be found by taking the

difference between two volume sources: one extending from the ground surface to infinite

depth and one extending from depth z to infinity (Fig. 2).

Fig. 2. The geometry of a slab source, extending from the soil surface to depth z.

The fluence rate from each of the two volume sources is given by Eq. 8 and the difference is

then given by