Journal of Nuclear and Radiochemical Sciences, Vol. 7, No.1, pp. R19-R23, 2006

Recent Advances in Radiochemical Studies of Low and Intermediate Energy Nuclear
Reactions Induced by Light Particles
S. M. Qaim*
Institut für Nuklearchemie, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany
Received: October 23, 2005
Radiochemical methods are of considerable significance in studies of low and intermediate energy nuclear reactions
induced by light particles, particularly when low-yield and soft-radiation emitting products are involved. A brief
description of the pertinent experimental techniques and nuclear model calculational codes is given. The data related
to nucleon emission, complex particle emission and formation of isomeric states are reviewed. The nuclear model
calculations can reproduce the nucleon emission cross sections fairly well. In the case of isomeric cross sections thecalculations demand a careful choice of the input parameters, especially regarding the level structure of the productnucleus. The emission of complex particles is as yet not described well by the theory; the radiochemically determined data serve to develop systematic trends which qualitatively enhance our knowledge on those processes.

1. Introduction
Studies of nuclear reactions in the low and intermediate
energy regions are of considerable importance for testing
nuclear models and for practical applications. It is known thatin the energy region below 10 MeV, compound nuclear processes dominate. Between 10 and 30 MeV the contribution ofpre-compound reactions increases and at energies above 30
MeV it becomes appreciable. At energies above 50 MeV till
about 150 MeV, a transition occurs from pre-compound tospallation, a region which is relatively less understood. Beyond
200 MeV the spallation process dominates. Thus systematicstudies of nuclear reactions from their thresholds up to about
200 MeV, covering emission of various types of radiation andparticles (photons, nucleons, complex particles, etc.) should
further enhance our understanding of the various nuclear theories involved. Another important fundamental aspect, attracting considerable attention in recent years, has been theformation of isomeric states in product nuclei.1 The isomericcross-section ratio for a pair of isomeric states is known todepend on the spins of the isomers concerned, as well as on thespins of the higher lying levels feeding the isomers.2 A studyof the isomeric cross-section ratio as a function of the incident
particle energy, especially near the threshold of a reaction,
should therefore lead to useful information on the spin-cutoff
parameter as well as on the level structure of the productnucleus. As far as practical applications are concerned, thecross section data are needed for calculations on neutronics,
activation and heat generation in nuclear technology,3 as well
as for production of high-purity medical radionuclides, especially at cyclotrons.4
Nuclear reactions are generally studied by physical techniques involving on-line recording of emitted radiation andparticles. Since with those techniques both angular and energy
distributions of the emission products can be determined, theresults lead to very useful information on the reaction mechanism.5 The other way of investigating a nuclear reaction is to
identify the reaction product by an off-line method. For an
unambiguous characterisation of the product often a radiochemical separation is needed.5 The product itself could be
identified via measurement of its radioactivity or, in some spe*Corresponding author. E-mail: s.m.qaim@fz-juelich.de Fax: +49
2461 612535

cial cases, via mass spectrometry (MS) 6 and/or accelerator
mass spectrometry (AMS).7 Obviously the off-line method,usually known as the “radiochemical method”, cannot yield thesame amount of information on the reaction mechanism as thephysical technique which involves recording of the emission
spectra. Resort is made here to comparisons of experimental
data with the results of nuclear model calculations and, therefrom, to deduce mechanistic information. This paper reviews
some of the recent studies performed using the radiochemical
technique. This approach is particularly useful when thematrix activity is strong and the desired reaction product is
weak. The technique is also very advantageous when the reaction product emits very soft radiation (e.g. low-energy β --particles or X-rays). Furthermore, for practical applications, theintegral data obtained via the radiochemical technique areoften more useful than the differential data determined via online emission spectrum measurement techniques.
2. Experimental Methods
2.1. Sample preparation and irradiations. Samples ofvery high-purity and well-defined chemical composition areneeded, particularly when low-yield reactions are under investigation. In work with neutrons, solid, liquid, or gaseous material packed in polyethylene, aluminium, or quartz can be
irradiated. In measurements with charged particles, on theother hand, relatively thin samples are needed. They are prepared by methods like sedimentation, vacuum evaporation,
electrolytic deposition, etc.
Irradiations and projectile flux determination demand considerable care since in cross section determination quantitative
measurements are involved. Monoenergetic neutrons in theregion of about 1 MeV are produced via the 7Li(p, n)7Be reaction, those in the range of 3 to 12 MeV via the 2 H(d, n) 3He
reaction, and the ones in the energy range of 13 to 20 MeV via
the 3H(d, n) 4He reaction. The low-energy deuteron beam (<500 keV) is produced in a Cockroft-Walton type generator, andthe higher energy beam is generally furnished by a Van deGraaff machine or a cyclotron. Irradiations with charged particles are done using beams extracted from cyclotrons or otheraccelerators. Generally thin samples placed in a row are irradiated together (“stacked-foil technique”). Due to energy degradation in the matter, the effective projectile energy in eachsample is different. It is thus possible to determine cross sec-

© 2006 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 06/30/2006

R20 J. Nucl. Radiochem. Sci., Vol. 7, No. 1, 2006
tions at several energies in just one experiment. In radiochemical studies, involving both neutrons and charged particles, theprojectile flux is measured using suitable monitor reactions.8,92.2. Radiochemical separations. Specific radiochemical
separations facilitate the isolation of transmutation products.
A variety of radiochemical methods like precipitation, co-precipitation, solvent extraction, ion-exchange chromatography,
thermochromatography, distillation etc. have been employed.
Some of the recent examples include the study of (p, 7Be) reaction on several transitional metals via high-performance ionchromatographic separation of beryllium,10 the 92Mo(α, 2n) 94Ru
process via thermochromatography,11 the short-lived isomericpairs 52m,gFe and 53m,gFe via the solvent extraction technique,12and the 58Ni(n, α)55Fe and 63Cu(n, p) 63Ni processes via the ionexchange method.13,14 For the separation of some reaction
products, a combination of several separation methods/techniques may be essential. A good example is the separation of45
Ca from Ti irradiated with protons.15 In addition to theremoval of the matrix activity, the radiochemical separations
also facilitate the preparation of thin sources for β --counting orsoft X-ray spectrometry. The excitation functions of the50
Cr(n, np+d) 49V and 58Ni(n, α) 55 Fe reactions, for example,
were determined via measurement of the 4.51 and 5.89 keV Xrays of radiochemically separated 49V and 55Fe, respectively13.
Similarly the 48Ti(n, α) 45Ca, 45Sc(n, p) 45Ca, natTi(p, x) 45Ca and63
Cu(n, p) 63Ni reactions were studied by radiochemical separations of the soft radiation emitting 45Ca and 63Ni which were
subsequently characterised by β - -counting. In studies on tritium emission, on one hand the residual nuclide, if radioactive,
was radiochemically separated and assayed and, on the other,
the accumulated tritium was separated and counted.16
2.3. Measurement of radioactivity. For characterising relatively strong reaction products, conventional γ-ray spectroscopyor β --counting was used. In investigations on low-yield reaction products, however, low-background detectors were
employed. In the case of low-energy γ -rays, small-sized Ge
detectors (with Be-windows) and, for X-rays, Si(Li) detectors
have been commonly used. The very soft β --emitters (like tritium) and the very soft X-ray emitter (like 37Ar) were determined via low-level gas counting.16
2.4. Other methods of product assay. In addition to measurement of radioactivity, some of the products could be identified
better by mass spectrometry (MS) or accelerator mass spectrometry (AMS). These techniques are especially suitable
when the transmutation product is stable or very long-lived.
Mass spectrometry has been occasionally applied to the determination of inert gases, like 3He and 4He, formed in the interactions of neutrons with various elements.6,16 The accelerator
mass spectrometry involves mass separation after accelerationof ions to several MeV energies and has been successfully
applied to the determination of cosmogenic nuclides like 26Al
and 36 Cl, environmentally related nuclide 14 C, and the highenergy nuclear fragmentation product 10Be.17,18
3. Nuclear Model Calculations
Low energy nuclear reactions are commonly treated in termsof the statistical model, generally using the Hauser-Feshbachformalism, which takes into account the angular momentum of
the evaporated particle and the level structure of the productnucleus. Later the pre-compound effect was also introduced
and the relevant codes GNASH (in USA) and STAPRE (inEurope) in several versions have been very successfully utilized
over the last 25 years in evaluating excitation functions, especiallyof neutron induced reactions up to 20 MeV.
Above 20 MeV the pre-compound effect becomes increasingly important and at energies above 50 MeV, it plays a dominant role. A very commonly used code in the intermediate
energy region was ALICE, developed by Blann about 30 years

Qaim
ago. Recently the Obninsk group introduced several modifications and termed it as the code ALICE-IPPE.19 The incorporated modifications include treatment of the level density in a
sophisticated way and consideration of the pre-equilibrium
cluster emission (d, t, and 4He). The code has been successfully applied to the calculation of excitation functions of a large
number of reactions.
In recent years a further calculational code, namely EMPIRE
II, has also been introduced.20 It combines the general featuresof the statistical process, the pre-equilibrium exciton modeland the various improvements mentioned above. Its use has as
yet not been so extensively tested as in the case of the other two
calculational codes.
4. Survey of Experimental Data, Comparison with
Theoretical Calculations, and Discussion
Extensive experimental and theoretical studies have been
performed in the low-energy region up to about 20 MeV, especially in the case of neutron induced reactions. With theincreasing incident particle energy, the experimental databasebecomes weaker. At energies between 100 and 200 MeV, forexample, very few data points are available. Nuclear model
calculations are fairly successful up to about 100 MeV. The
various nuclear processes are considered below separately.
4.1. Emission of nucleons. In the low and intermediate
energy regions, the emission of nucleons has been extensively
studied, mainly because the reaction cross sections are high.
Many of the reaction products have been identified without
radiochemical separations, except for cases where β --emittersor low-energy X-ray emitters are involved. With neutrons,studies are generally limited up to 20 MeV and the reactions ofinterest are (n, xn), (n, pn), etc. For theoretical analysis mostly
the codes GNASH and STAPRE have been used. In the caseof protons, measurements have been performed often up to 100
MeV, the emphasis being on (p, xn) and (p, pxn) processes.
Besides GNASH and STAPRE, considerable use has been
made of the code ALICE-IPPE to calculate the excitation functions. Some recent studies on the target elements Cu, Zn, Rb,
Te, Pb, etc. show that both the (p, xn) and (p, pxn) processes
can be described very well by the model calculations,21-25 provided the input parameters are properly chosen. The same istrue for (d, xn), (3He, xn) and (α, xn) processes where, however,more care in parameter choice is necessary.26-30 In Figure 1 areshown, as typical examples, the experimental results on the(p, xn) processes on 85Rb and 122Te. In the former case calculations were done using the code ALICE-IPPE and in the latterthe code STAPRE. In both cases generally good agreement
was found between the experiment and theory, the latter code
having also the capability of calculating the excitation functionof the isomeric state 120m I. The code ALICE-IPPE has someproblem in reproducing the simple (p, n) reaction at energies >30 MeV. In the study of neutron and proton induced reactions
recently the code EMPIRE II has also been used and the results
are more or less similar to those from the other codes. Between
100 and 200 MeV the experimental database is rather weak andthe differences between the experimental data and the resultsof nuclear model calculations become increasingly significant.
Similarly when the number of emitted nucleons becomes large,(i.e. several neutrons and several protons) or when an unusual
reaction like (p, 2p) occurs, the deviations between the experiment and model calculations may increase.22,23
4.2. Emission of complex particles. In contrast to nucleon
emission, the emission of complex particles (d, t, 3He, α, 7Li,
7Be, 10Be, etc.) has not been extensively studied, mainly due toboth experimental and calculational difficulties (for a detailed
review on this topic cf. Ref. 16). This is an area of researchdemanding extensive use of radiochemical separations. Tritiumand 3He emission was studied radiochemically mainly in neutron

Recent Advances in Radiochemical Studies of Low and Intermediate

J. Nucl. Radiochem. Sci., Vol. 7, No. 1, 2006 R21

Coss section/mb

Coss section/mb

A. 85Rb(p, xn)85,83,82,81SrProton energy/MeV

Coss section/mb

B. 122Te(p, xn)122,121,120m,gI

Proton energy/MeV
Figure 2. Systematics of (p, 7Be) excitation functions on target elements C to Bi (taken from Ref. 31).

Figure 1. Excitation functions of (p, xn) reactions on (A) 85Rb and(B) 122Te, measured via the activation technique. The experimental
data for 85Rb are shown as smooth solid curves and the calculational
results from the code ALICE-IPPE as dashed curves. For 122Te, theexperimental results are depicted by symbols and the values from thecode STAPRE as solid curves (results taken from Refs. 24 and 26).

induced reactions using tritium counting and mass spectrometry(cf. Ref. 16). The α-particle emission data were obtained both
via mass spectrometry17 and measurement of the reaction products. In the case of 7 Be and 10Be, radiochemical separations
followed by γ -ray spectrometry31 and accelerator mass spectrometry,18 respectively, have been applied. As far as nuclear
model calculations are concerned, there is as yet no satisfactory
method of estimating the complex particle emission cross section, the exceptions being triton and α-particle emissions. In
the former case the statistical process has been partly successful
and, in the latter, a combination of statistical and pre-compoundprocesses has successfully reproduced the experimental data.
In other cases development of systematic trends has proved tobe very advantageous in describing some characteristics of
those reactions. As an example, some results for the (p, 7Be)
reaction31 are shown in Figure 2. In the light mass region, 7Be
is expected to be formed as a product nucleus. For medium
and heavy mass target nuclei, however, the emission of thecomplex particle (7Be) is most probable. The systematic trendsof the excitation functions reveal that for proton energies above
40 MeV the emission of 7Be decreases with the increasing mass ofthe target nucleus. The trend is also valid for the heavy elements(Au and Bi) which undergo fission with intermediate energy
protons. In a similar recent radiochemical study18 the target
dependence of 7Be and 10Be emission in neutron and α-particle
induced reactions was studied, the former product via γ-ray
spectrometry and the latter via AMS. The results for α-particle
induced reactions at 400 MeV are shown in Figure 3. Evidently,
the heavier targets show a preferential emission of the 10 Be
fragment over 7 Be, probably due to the increasing N/Z ratiowith the increasing mass of the target nucleus.

Reaction coss section/mb

Proton energy/MeV

Target mass
Figure 3. Yields of Be and Be in 400 MeV α-particle induced
reactions shown as a function of the target mass (taken from Ref.
18).
7

10

4.3. Formation of isomeric states. In comparison to thetotal cross section of a reaction channel, the partial cross section describing the population of a particular isomeric state ismore difficult to measure and to calculate (for a detailed review
on this topic cf. Ref. 1). Due to emission of soft radiation by
the low-lying levels involved, radiochemical methods of separation and thin target preparation are ideally suited for themeasurement of isomeric cross sections. In recent years extensive investigations have been performed on several isomericpairs, e.g. 52m,g Mn, 58m,g Co, 73m,g Se, 94m,gTc, 120m,g I, 197m,g Hg,
etc.,2,26,32-36 involving different combinations of target, projectile, ejectile, and level structure of the product nucleus. Nuclear
model calculations have been successfully performed using thecode STAPRE and partly EMPIRE II (both incorporating statistical and pre-compound models) in combination with a very
careful choice of the input parameters (optical model parameters, level structure of the product nucleus, branching ratios ofγ -rays from discrete levels, spin dependence of the level density
distribution, etc.). The results for the formation of a typicalR22 J. Nucl. Radiochem. Sci., Vol. 7, No. 1, 2006
isomeric pair
Co in the Ni(p, α)-reaction are shown in
Figure 4. The metastable state has a spin value of (5+) and theground state (2+). The characterisation of the low-energy transition (E =25 keV) was facilitated by the radiochemical method.
Evidently, the probability of formation of the high-spin isomerincreases rapidly with the increasing kinetic energy of the projectile. The experimental data are reproduced approximatelyby the STAPRE calculation, especially when the η value (ratioof Θeff to Θrig) is properly chosen. In a recent study35 the formation of the isomeric pair 52m,g Mn was investigated in fivenuclear processes and the results are shown in Figure 5. The
important role of the spins of the two states involved is confirmed. The cross section of the low-spin isomer in comparison to that of the high-spin ground state initially decreases with
the increasing incident projectile energy, but becomes almost
constant at high excitation energies. The (p, n) and (3He, t)
reactions (curves A and B) occur on the same target nucleus
and the product studied is also the same. The same is true forthe (d, α), (n, t) and (3He, αp) reactions (curves C, D, and E).
The magnitudes of the cross-section ratios, however, differ
considerably. It is concluded that the reaction channel affects
the isomeric cross-section ratio appreciably, particularly when
the channels differ widely, e.g. (p, n) and (3He, t) processes.
Very recently the formation of high spin isomers 195m Hg and
197m
Hg was studied in several reactions.36 A description of theisomeric cross-section ratio by the model was possible only
61

32

σm /(σm +σg )58m,gProton energy/MeV

σm /(σm +σg )Figure 4. Isomeric cross-section ratio for the 61Ni(p, α) 58m,gCo process as a function of proton energy. The nuclear model calculation
was done using the code STAPRE for two values of η (ratio of Θeff toΘrig) to demonstrate the effect of spin distribution of level density(taken from Ref. 32).

Energy/MeV

Figure 5. Experimental isomeric cross-section ratios for the formation of 52m,gMn in several nuclear reactions, plotted as a function of
the incident particle energy (taken from Ref. 35).

Qaim
with a very low value of η, i.e. the Θeff to Θrig ratio. On the basisof those results, a mass dependence of η has been proposed.
5. Conclusions
The radiochemical method of nuclear reaction cross section
measurements is commonly used in low and intermediate
energy regions. It is especially advantageous for studies oncomplex particle emission and is ideally suited for measurementof isomeric cross sections. In the case of nucleon emission, theexperimental total and partial cross sections of a reaction channel
are reproduced fairly well by a combination of the statisticaland pre-compound models, provided the input parameters areproperly chosen. In contrast, the complex particle emission is
not described properly by those models, presumably due tostrong contributions from direct processes. Further experimental
and theoretical work in this direction is necessary.
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