Applications of photonuclear reactions

314

Nuclear Instruments and Methods in Physics Research B50 (1990) 314-320 North-Holland

APPLICATIONS

OF PHOTONUCLEAR

REACTIONS

D.J.S. FINDLAY

The potential use of photonuclear reactions for a range of applications is described. These are: photonuclear transmutation doping of semiconductors, neutron production from electron linacs, quality checking of radioactive waste, fission product indneration, photoexcitation of isomers for dosimetry, and nuclear resonance fluorescence for materials analysis. Initial brief descriptions of atomic and nuclear interactions of photons and of bremsstrahhmg are given.

1. Introduction This paper nuclear of

describes

reactions

photon

several

applications

for a variety of purposes.

activation

is not

considered,

of photoThe subject as

this has

reviewed [l]. The paper is, however, intended to give a flavour of applications of photonuclear reactions in fields other than those of surface materials analyses generously covered in other papers at this conference. The paper begins with a brief description of some of the basic interactions of photons (both atomic and nuclear), and progresses through ~nsideration of photon sources and bremsstrahhmg spectra to descriptions of six examples of the applications of photonuclear reactions for a range of different purposes. recently

been comprehensively

2. Basic photon interactions Photons are stable, massless elementary particles with spin J = 1. The projection of the spin, mJ, can be f 1, ~~espond~g to left-hand or right-hand circular polarisation. An unpolarised photon beam simply has equal numbers of left-hand- and right-hand-polarised photons. The fact that photons are massless means that a photon’s momentum is proportional to its energy, a relationship very different from that of momentum proportional to the square root of the energy corresponding to nonrelativistic charged particles. A nuclear interaction of a 10 MeV photon brings a momentum of 10 MeV/c to the reacting system; a 10 MeV proton would bring in 137 MeV/c. For the present purposes, relevant photon energies are - 3-30 MeV, and there are several characteristics of photons in this energy range which must be borne in mind when applications of photonuclear reactions are considered. First, there are no primary sources of photon beams. Instead, photons are always produced as

secondary beams, in general from electron beams. Secondly, photons experience both atomic and nuclear interactions. A consequence of the relatively weak atomic interaction is the high penetrating power of photons in materials. A consequence of the nuclear interaction is that in photonuclear reactions product nuclei can never be heavier than target nuclei, whereas heavier nuclei can be produced in charged particle reactions such as the (d, p) reaction, Thirdly, it is clear that it is not possible to focus photon beams magnetically, although focusing by means of bent crystals is used for non-applications purposes. Fourthly, photons cannot be detected directly; photons must be converted fist to charged particles. 2.1. Atomic interactions Within the present context, the simplest manifestation of the interaction of photons with atoms is the attenuation of a photon beam as it penetrates bulk material. Neglecting the effects of scattered photons and secondary electrons and positrons (effects which in practice tend to reduce the effectiveness of bulk gamma-ray shielding), a photon beam undergoes exponential absorption within bulk material according to I(x) = I(0) exp( - px), where x is the distance within the material and J.J is the linear attenuation coefficient. The attenuation coefficient is often expressed as the mass attenuation coefficient p/p, where p is the density of the bulk material. For all materials, the energy dependences of p/p are very roughly similar, having minima of - 0.03 cm2/g somewhere between - 2 and - 20 MeV where Compton scattering dominates, and rising monotonically both below this ~~~, where photoelectric absorption begins to dominate, and above, where pair production in the Coulomb field of the nucleus begins to dominate. A useful table of attenuation coefficients is ref. [2]. As examples, some l/e attenuation lengths are shown in table 3, and in ad-

D.J.S. Findlay / Applications

10 MeV gamma I/e attenuation length [cm] 45 17 4.3

Water Si Fe

Range in Si

Particle

[cm] 10 MeV 10 MeV 10 MeV 100 MeV

gamma ektron Proton

17 2.4 0.07

fission fragment

0.002

dition the high penetration of photons is emphasised by the comparison with the ranges of various charged particles. 2.2.

Nuclear

Reaction channel

Example

(Possible) application

(Y, Y)

resonance fluorescence

bulk materials analysis

photoexcitation of isomers

dosimetry

photon elastic scattering

(Y, Y’) photon inelastic scattering

1.8

Pb

315

reactions

Table 2 Some photonuclear reaction channels

Table 1 l/e attenuation lengths and ranges Material

of photonuclear

photoprotons

transmutation doping of semiconductors

(y, n)

photoneutrons

hnac neutron sources, fission product incineration

(y, f)

photofission

actinides in radioactive waste

as a function

of the photon energy k is shown in fig. 1. In an attempt at rough universal applicability, the ordinate is shown as the cross section per nucleon utoz(k)/A, where A is the mass number of the nucleus [4J. The dominant feature of utot is the hump corresponding to the giant resonances [S], the most important of which is the giant dipole resonance (GDR) in which the protons in the nucleus oscillate as a whole against the neutrons as a whole. The energy of the GDR

interactions

Interactions of photons with nuclei are reflected in photonuclear cross sections. A very useful and very comprehensive set of photonuclear cross-section graphs is given in ref. [3]. A schematic illustration of the energy dependence of the total photonuclear cross section Q,,

100

(y, p)

10

R (F)

I

2-

Is 3 Phclopion threshold

BOUND-STATES

FOR MATI 0 N

Fig. 1. Schematic representation of the total photonuclear cross section per nucleon. The characteristic interactions in different energy regions are shown. The representation is not good for very light nuclei. The scale along the top is the reduced wavelength of the photon in fm. V. VARIOUS ~CHN~QUES

316

D.J.S. Findlay / Applications

is given roughly by [S] EGDR = 80A-‘/3, and a typical value is EGDR = 20 MeV. (For deformed nuclei, such as the actinides, the GDR splits into two components corresponding to oscillations along the different semiaxes.) The strength of the photonuclear interaction is given by the sum rule [S] /0

- 3o MeV~tot( k ) d k = 60 NZ/A

MeV mb .

Since N = Z = A/2, the right-hand side of this equation is consistent with the l/A dependence in fig. 1. In general there are many photonuclear reaction channels, the sum of the cross sections for which is uiol. Some of these are given in table 2. The (possible) applications listed are discussed in section 4.

3. Photon sources As mentioned in section 2, photon beams, are never produced as primary beams, only as secondary beams, the primary particles being electrons and positrons. Electron beams from accelerators can be used for photon production through bremsstrahlung, and through more esoteric techniques such as photon tagging [6] and backward Compton scattering [7]. Positron beams can be used to produce quasimonochromatic photons from annihilation in flight [8]. All these techniques are used for making photonuclear cross-section measurements, but bremsstrahlung is probably the only practical method for realistic applications of photonuclear reactions. An outstanding review of bremsstrahlung, in spite of its age, is that of Koch and Motz [9]. Nowadays, the practicality of partial wave techniques has led to a supersession of the Coulomb correction techniques described in ref. [9], and a useful modem tabulation of bremsstrahlung cross sections has been given by Seltzer and Berger [lo]. The basic bremsstrahlung process is the acceleration of an electron in the Coulomb field of a nucleus (a representative value for which is - 1016 V/cm). The basic bremsstrahlung cross-section differential in photon energy and angle, d2a/dk d0, is proportional to the square of the atomic number Z of the nucleus, which is of course why bremsstrahlung converter targets (radiators) are in general constructed from high-Z materials. Since in practice power dissipations of - 1 kW/cm’ can easily be obtained from electron accelerators, converter targets are often constructed as a series of water-cooled thin sheets. Bremsstrahlung photons from a converter target are in general very forwardly directed. For thin converter targets, in which the electron energy loss in the target is small compared with the incident electron energy, the bremsstrahlung spectrum integrated over all photon directions has a shape very similar to the shape of the

of photonuclear reactions

integrated-over-angle (IOA) bremsstrahlung cross section da/dk. To within a factor - 2 for rough practical purposes, the IOA bremsstrahlung intensity spectrum for a thin target can be taken to be rectangular,

where dN/dk is the number of bremsstrahlung photons per MeV per electron of energy E incident on the target, x is the thickness of the target, and X0 is the radiation length of the target material (X0 [g/cm21 = 180A/Z2). For the bremsstrahlung spectrum in the forward direction (i.e. at 0” ) another useful rough approximation is

where d2N/d k d0 is the number of photons per MeV per steradian per electron, and k and E are in MeV, obtained by taking the electron multiple-scattering distribution to be uniform up to the rms angle (and to be zero beyond this) and integrating over the target thickness. The radiation dose in a bremsstrahlung beam may be estimated from [2] dose = uE$&(k)ky(k) /

dk,

where the dose is in MeV/g, d2N/d k dS is the number of photons per MeV per cm=, and p&p is the mass-energy-absorption coefficient [2]. By taking pL,,/p = 0.02 cm2/g, an approximation good to a factor - 2 for all materials, setting d2N/dk dS = d2N/dk da/ D2 and using eq. (2), a useful formula for the radiation dose rate in the forward direction is doserate=1.4x106s

ln(-$$$),

(4)

where the dose rate is in rad/s, D is the distance from the target [cm] and Z is the electron current [A] incident on the target. For thicker converter targets in which the electron energy loss is a significant fraction of the incident electron energy or in which the electron beam stops, the situation is more complicated. While coupled photon-electron(positron) computer programs such as ETRAN or EGS are available for comprehensive computation of bremsstrahlung spectra [11,12], it is sometimes useful to have a relatively easily evaluable analytic expression for the spectrum d’N/d k dS2 (x, E, k, 8) of the number per MeV per steradian per electron of bremsstrahlung photons with energy k and angle 8 (relative to the direction of the incident electron) for electrons of energy E incident on a target of thickness x. A useful expression, intended for energies up to the giant resonance region, is given in ref. [13]. This expression, involving the exponential integral, is

D.J.S. Findlay / Applications of photonuclear reactions

317

in natural silicon) being transmuted by thermal-neutron capture to 31Si nuclei which then decay to 31P nuclei, thereby leading to a distribution of phosphorus throughout the silicon. However, NTD can only be used to produce n-type silicon. Photonuclear transmutation doping (PTD) can be used to produce aluminium-doped p-type silicon through the following reactions:

good to - 20% for any thickness, photon energy and angle. A quantity often of practical interest is the bremsstrahlung conversion efficiency Y,, which is the ratio of power in the bremsstrahlung beam to power in the electron beam. For high-Z targets Yy varies from - 8% at 5 MeV (the highest energy likely to be permitted for y-ray irradiation of food) to - 70% at 100 MeV. (Older values of Y,, e.g. as given in ref. [9], tend to be overestimates at lower energies because of the particular simplifying approximations made.)

**Si( y , P)~‘A~, *‘Si( y, n)27Sis27Al, and the production and characterisation of PTD silicon are described in ref. [15]. Discs of silicon 20 mm in diameter and 1 mm thick were irradiated with 25 MeV bremsstrahlung from the Harwell electron linear accelerator (linac) HELIOS as shown in fig. 2. By folding the bremsstrahlung spectrum (calculated according to ref. [13]) with the sum of the photoneutron and photoproton cross sections, a concentration of 5 X lOi Al acceptors/cm3 was predicted (with an error of - 20%). Photoluminescence measurements gave very satisfactorily agreeing values of - 6 X lOi cme3, and electrical measurements were also in satisfactory agreement. Using a 1 mA, 25 MeV electron beam from a typical high-power electron Iinac, the time to produce 1 g of PTD silicon with 1014 Al acceptors/cm3 would be - 20 s. Assuming that current uncertainties over annealing procedures can be resolved, possible uses for PTD silicon could include: high-voltage, high-current devices; p-n junction semiconductor radiation detectors; resis-

4. Applications In this section, six examples are given of the application of photonuclear reactions. The first three applications are current; the second three are more speculative. 4. I. Photonuclear transmutation doping of semiconductors In particular circumstances it can be important for semiconductor material to be very uniformly doped (e.g. for large high-voltage high-current thyristors). One way of achieving this is to arrange for a uniform flux of nuclear particles to be present throughout bulk semiconductor material (e.g. an ingot) and to transmute some of the host atoms to dopant atoms. Neutron transmutation doping (NTD) of silicon is a well established process (e.g. ref. [14]), %i nuclei (present to 3%

6 cm II II II

4 0.6mm Ta radiator 2

0

25 MeV electron

-2

0.5mm Al scatterer

beam

-4

II

II

-6 Cooling water

I

I

-12

-10

I -8

I

I

I

-6

-4

-2

I 0

I 2

I 4

I 6

I 8

I 10

I 12 cm

Fig.

2. Outline of the experimentalsystem used for photonuclear transmutationdoping of semiconductors.The aluminium scatterer is to smooth out any spatial inhomogeneities in the electron beam, and the aluminium stopper is to prevent residual electrons from reaching the silicon samples.

V. VARIOUS TECHNIQUES

318

D.J.S. Findlay /Applications of photonuclear reactions

tivity trimming, e.g. by increasing the resistivity of NTD silicon; uniform-resistivity p-type Si infrared bolometers. Other semiconductors can be doped by PTD, such as diamond which can be made p-type using the ‘*C(y, p)“B reaction to produce boron acceptors, and therefore also SIC. While PTD of Ge, GaAs, etc. is possible, induced radioactivity problems may inhibit use for all but the most special&d purposes. A further use of PTD could be for the production of uniform and well characterised calibration samples of trace impurities. Such samples could probably find useful application in the materials analysis techniques described elsewhere in this conference.

‘O7------

4.2. Linac neutron targets Neutron sources based on photonuclear reactions driven by bremsstrahlung from electron linacs can have a wide variety of applications. These include: neutron interrogation, for fissile material in radioactive waste 1161, for uranium enrichment measurements [17], and for economically valuable components of ores in mining; neutron radiography; and to drive a lead slowingdown spectrometer, a unique high-intensity source of quasi-monochromatic neutrons. While there are many existing predictions of photoneutron yields from linac targets, there are not so many measurements, especially at lower energies, and some of the approximations made in the predictions are not particularly realistic. For energies up to - 15 MeV, neutron-producing targets of 2H and Be driven by high-Z bremsstrahlung converter targets give the best yields (2H and 9Be have the low photoneutron threshold energies of 2.2 and 1.7 MeV respectively). For energies above - 15 MeV, heavymetal targets such as Ta and “a’U give greater outputs. In ref. [18] are described calculations of the neutron production rates from heavy-water and beryllium targets. The method of calculation was the integration of the product of the bremsstrahlung spectrum from the converter target calculated according to ref. [13] as a function of position and the 2H( y, n) and ‘Be( y, n) cross sections throughout the neutron-producing target volume, including the effects of photon attenuation. Some results are shown in fig. 3, and are significantly lower than the predictions of Bowman [19] made using unreasonable simplifying assumptions. Time-of-flight measurements supporting the present results are described in ref. [20]. 4.3. Quality checking of radioactive waste An application of photonuclear reactions in which the high penetrating power of the photons is exploited is the assessment of the total actinide content of 500 1 drums of cemented intermediate-level radioactive waste. Such assessment could be required before the waste

-

10'

’

0

I

I

I

5

10

15

Electron

Energy

I 20

, 25

0.1eV)

Fig. 3. Neutron output from heavy-water and beryllium linac targets. Both neutrons per pC of electron charge and per joule of electron energy incident on the bremsstrahlung converter target are shown.

drum is finally placed in a repository. A suitable measurement technique is described in ref. [21], and is shown schematically in fig. 4. An energy-analysed electron beam from an accelerator is directed on to a converter target, and the resulting bremsstrahlung illuminates and penetrates the 500 1 drum. The bremsstrahlung photons induce photofission and photoneutron reactions in any residual quantities of actinide isotopes in the drum resulting, for example, from incomplete dissolution of nuclear fuel during reprocessing, and the resultant fission neutrons and photoneutrons are counted in thermal neutron detectors placed around the drum. Although the attenuation of the bremsstrahlung through and the variation in neutron detection efficiency across a drum diameter, nominally 75 cm, are factors of - 100 and - 1000 respectively, the variation in overall response of the measurement system is minimised by adopting an asymmetric

D.J.S. Findlay / Applications of photonuclear reactions

I i5 r C

319

used to photoproduce neutrons from the deuterium alone, since 5 MeV is well below B, and S,. Of course, neutron radioactivity again is counted. Thirdly, measurements are made of the neutron radioactivity alone in the absence of bremsstrahlung. By combining the results of the three measurements, the actinide contribution can be extracted. Since photofission and photoneutron cross sections are roughly similar for all actinide isotopes, the measurement technique is roughly sensitive to all actinides, and therefore values of the total actinide content of the drum can be obtained. Some results are reproduced in fig. 4, from which it can be seen that the measured response curve is uniform to within an overall factor of 1.4. The estimated total actinide sensitivity for the - 1 t drum for a practical plant system is - 30-100 g, which corresponds to - 1 g sensitivity for fissile isotopes for typical few-percent-enriched fuel if the fuel history is known.

radiator stopper

4.4. Photonuclear fission product incineration

0 -1

10

20

30

40 cm

2

7 MeVm’U response function f

measured

12 z 0, u

Fig. 4. General schematic arrangement for quality checking of radioactive waste packages using photofission and photoneutron reactions. The predicted and measured response functions in the midplane of the drum for 7 MeV endpoint bremsstrahlung are also shown as functions of radial position within the drum (0 cm corresponds to the centre of the drum).

arrangement in which the source of the interrogating photons and the detectors for the signature radiation are at opposite sides of the drum. Three separate measurements are made. First, 7 MeV endpoint bremsstrahlung is used to induce photofission and photoneutron reactions in any actinides present in the drum. This endpoint energy is both above the fission barrier B, = 6 MeV and the photoneutron threshold energy S,, = 6.0-6.5 MeV in actinides. However, although 7 MeV is below S, for all nuclei likely to be in the cement matrix, neutrons will still be photoproduced from the deuterium naturally present in the hydrogen in the water in the cement. Further, neutrons will also be counted from neutron radioactivity due to ((Y, n) reactions and spontaneous fission in the waste drum. Secondly, - 5 MeV endpoint bremsstrahlung is

Fission products extracted from spent nuclear fuel in a reprocessing plant form a very radioactive waste stream. To try to reduce this radioactivity, nuclear incineration has been considered (e.g. ref. [22]). While most fission products can in principle be incinerated using neutrons in a reactor, low thermal-neutron capture cross sections lead to ineffective neutron incineration of the particular fission product isotopes %r and t3’Cs (half-lives 28 and 30 yr, respectively). However photonuclear incineration of ‘%r and i3’Cs has been proposed by Matsumoto et al. [23] using the photoneutron channel and - 20 MeV photons. It is insufficient to consider only the (y, n) reactions leading to 89Sr and 136Cs (half-lives 50 and 13 d, respectively), and Matsumoto et al. include all possible (y, n) reactions and l3- and l3’ decays. The results indicate that fluxes of - 1018 photons/cm* s are required. However, the generation of such fluxes is far beyond the capabilities of present-day accelerators, and it may be noted that a 1 mA 30 MeV beam from a high-power electron linac produces - 1013 - 20 MeV photons/cm* s. 4.5. Isomer excitation for mixed y-ray and fast-neutron dosimetty Fast-neutron dosimetry using activation techniques requires the use of threshold reactions to eliminate sensitivity to thermal-neutron contamination which in practice can be very difficult to estimate. One such reaction is the ‘151n(n, n’) reaction with a 336 keV threshold leading to the production of the 4.5 h isomer 115mIn, the 336 keV y-ray from which is easily counted, e.g. using a Ge(Li) detector. However, if the fast neutrons are accompanied by y-rays, as will be the case if the neutrons are produced by an electron linac, then V. VARIOUS TECHNIQUES

320

D.J.S. Findloy / Applications of photonucleor reactions

photoexcitation of the llsrnIn isomer by the ‘151n(y, y’) reaction is also possible. To separate by y-ray and neutron components of the activity, further information is necessary. It is possible that such further information could be provided by the simultaneous excitation of another isomer such as the lllmCd isomer. However, while tabulations of neutron cross sections to isomers are available [24], there are little data on photoexcitation of isomers, especially at energies above several MeV (e.g. references in ref. [l]). If a series of isomer photoexcitation measurements were made with bremsstrahlung as a function of endpoint energy, then these cross sections could be used along with the neutron cross sections for the separation of y-ray and neutron components of radiation dose in mixed radiation fields.

applications are often very different from the applications of charged-particle reactions which, because of the (relatively) short ranges of the charged particles involved, are particularly suitable for materials surface analyses. Because the field of photonuclear physics is not as well populated as that of low-energy charged-particle nuclear physics, techniques for the application of photonuclear reactions are in general not so well known or so well advanced. However, it has been shown that, with suitable development of techniques, photonuclear reactions could be applied with advantage for a wide range of purposes.

References 4.6. Nuclear resonance fluorescence An established technique for materials analysis is X-ray fluorescence, which is sensitive mainly to the surface of a sample because of the large attenuation of the X-rays used. For analysis throughout the bulk of a thick sample, however, it is possible that the development of a technique based on nuclear resonance fluorescence could be used. The basis of the technique is the illumination of the sample with bremsstrahlung, the continuous energy spectrum in which can photoexcite nuclei from their ground states to (bound) excited states. The excited states then decay by y-ray emission, and these decays can be observed as lines on top of a continuous background from scattered bremsstrahlung in a y-ray spectrum observed with a Ge(Li) detector. Good collimation and beam hardening are necessary. While nuclear resonance fluorescence measurements using conventional electron linear accelerators have been made (e.g. ref. [25]), such measurements are made very difficult by the poor duty factor, typically at most 10e3, of such accelerators. However, with the introduction of 100% duty factor accelerators such as the Illinois [26] and Maim [27] microtrons, such measurements should become very much easier, and might allow nuclear resonance fluorescence to join the ranks of practical nuclear techniques for materials analysis.

5. Summary and conclusions In this paper, after initial discussion of photon interactions, photon sources and bremsstrahlung, six examples of applications of photonuclear reactions for photons in the - 3-30 MeV energy region have been presented. Most of the examples exploit the high penetrating power of photons. It is quite clear that the

[l] C. Segebade, H.-P. Weise and G.J. Lutz, Photon Activation Analysis (De Gruyter, Berlin, New York, 1988). [2] J.H. Hubbell, NSRDS-NBS 29 (Nat. Bureau of Standards, 1969). [3] Handbook on Nuclear Activation Cross Sections, Technical reports series no. 273 (IAEA, Vienna, 1987). [4] M.R. Se&, private communication. [5] R. Berg&e, Lecture Notes in Physics 61 (1977) 1. [6] J.D. KelIie et al, Nucl. Instr. and Meth. A241 (1985) 153. [7] L. Frederici et al., Nuovo Cim. 59B (1980) 247. [8] B.L. Berman and S.C. Fultz, Rev. Mod. Phys. 47 (1975) 713. [9] H.W. Koch and J.W. Motz, Rev. Mod. Phys. 31 (1959) 920. [lo] S.M. Seltzer and M.J. Berger, At. Data Nucl. Data Tables 35 (1986) 345. [ll] M.J. Berger and S.M. Seltzer, Phys. Rev. C2 (1970) 621. [12] RSIC Computer Code Collection, Oak Ridge, CCC-331. [13] D.J.S. Findlay, Nucl. Instr. and Meth. A276 (1989) 598. [14] J. Guldberg (ed.), Neutron Transmutation-Doped Silicon (Plenum, New York, 1981). [15] D.J.S. Findlay and H.J. Totterdell, Semicond. Sci. Tech. 3 (1988) 388. [16] T.V. Molesworth et al, Proc. BNES Int. Conf. on Radioactive Waste Management, Brighton, May 1989 (BNES, London, 1989). 1171 L.A. Franks et al., Nucl. Instr. and Meth. 193 (1982) 571. [18] M.R. Sent, to be published. [19] C.D. Bowman, Nucl. Sci. Eng. 75 (1980) 12. [20] Harwell report, AERE PR/NP 35. [21] D.J.S. Findlay, Harwell report AERE R 12863. [22] H.A.C. McKay et al., EUR 5801~ (1977). [23] T. Matsurnoto, Nucl. Instr. and Meth. A268 (1988) 234. [24] R. Kinsey, ENDF/B summary documentation, BNLNCS-17541 (ENDF-201) 3rd ed. [25] N. Shikazono and Y. Kawarasaki, Nucl. Instr. and Meth. 92 (1971) 349. [26] A. Gerard and C. Samour (eds.), Nucl. Phys. A446 (1985). [27] H. Herminghaus et al, Nucl. Instr. and Meth. 138 (1976) 1.