Low-energy (5-40 eV) electron-stimulated desorption of anions from physisorbed DNA bases

We present the results of experiments on anion desorption from the physisorbed DNA bases adenine, thymine, guanine and cytosine induced by the impact of low-energy (5-40 eV) electrons. Electron bombardment of DNA base films induces ring fragmentation and desorption of H(-), O(-), OH(-), CN(-), OCN(- ) and CH(2)(-) anions through either single or complex multibond dissociation. We designate the variation of the yield of an anion with electron energy as the yield function. Below 15 eV incident electron energy, bond cleavage is controlled mainly by dissociative electron attachment. Above 15 eV, the portion of a yield function that increases linearly is attributed to nonresonant processes, such as dipolar dissociation. A resonant structure is superimposed on this signal around 20 eV in the anion yield functions. This structure implicates dissociative electron attachment and/or resonant decay of the transient anion into the dipolar dissociation channel, with a minimal contribution from multiple inelastic electron scattering. The yields of all desorbing anions clearly show that electron resonances contribute to the damage of all DNA bases bombarded with 5-40 eV electrons. Comparison of the ion yields indicates that adenine is the least sensitive base to slow electron attack. Electron-irradiated guanine films exhibit the largest yields of desorbed anions.     

Sequence-specific damage induced by the impact of 3-30 eV electrons on oligonucleotides.

The ability of low-energy electrons to induce single- and double-strand breaks in DNA has recently been demonstrated. Here we show the propensity of 3-30 eV electrons to initiate base sequence-dependent damage to a short single DNA strand. Solid monolayer films of homogeneous thymidine (T(9)) and deoxycytidine (dCy(9)) and heterogeneous oligomers (T(6)dCy(3)) are bombarded with 1-30 eV electrons in an ultrahigh-vacuum system. CN, OCN and/or H(2)NCN are detected by a mass spectrometer as the most intense neutral fragments desorbing in vacuum. A weaker signal of CH(3)CCO is also detected, but only from oligonucleotides containing thymine. Below 17 eV, the energy dependence of the yields of CN, OCN and CH(3)CCO exhibits resonance-like structures, attributed to dissociative electron attachment (DEA). Above 17 eV, the monotonic increase in the fragment yields indicates that nonresonant processes (i.e. dipolar dissociation) control the fragmentation of these molecules. Within the energy range investigated, comparison of the magnitude of the total fragment yields produced by electron attack on dCy(9), T(6)-dCy(3) and T(9) suggests the following order in the sensitivity of single-strand DNA: dCy(9) > T(6)-dCy(3) > T(9). At 12 eV, the total fragment yields are found to be 5.8, 5.0 and 3.9 x 10(-3) fragment/electron, respectively. From the yields obtained with the two homo-oligonucleotides, we differentiate between contributions arising from the chemical nature of the base and the effect of environment (i.e. the sequence) when a thymidine unit in T(9) is replaced by dCy. The base sequence-dependent damage is found to vary with incident electron energy. These results reinforce the idea that genomic sensitivity to ionizing radiation depends on local genetic information. Furthermore, they underscore the possible role of low-energy electrons in the pathways responsible for the induction of specific genomic lesions.     

Alteration of protein structure induced by low-energy (<18 eV) electrons. I. The peptide and disulfide bridges

We present measurements of low-energy (<18 eV) electron-stimulated desorption of anions from acetamide (CH(3)CONH(2)) and dimethyl disulfide [DMDS: (CH(3)S)(2)] films. Electron irradiation of physisorbed CH(3)CONH(2) produces H(-), CH(3)(-) and O(-) anions, whereas the H(-), CH(2)(-), CH(3)(-), S(-), SH(-) and SCH(3)(-) anions are observed to desorb from the DMDS film. Below 12 eV, the dependence of the anion yields on the incident electron energy exhibits structures that indicate that a resonant process (i.e. dissociative electron attachment) is responsible for molecular fragmentation. Within the range of 1-18 eV, it is found that (1.7 and 1.4) x 10(7) H(-) ions/incident electron and (7.8 x 10(-11) and 4.3 x 10(-8)) of the other ions/incident electron are desorbed from acetamide and DMDS films, respectively. These results suggest that, within proteins, the disulfide bond is more sensitive to low-energy electron attack than the peptide bond. In biological cells, some proteins interact closely with nucleic acid. Therefore, the observed fragments, when produced from secondary low-energy electrons generated by high-energy radiation, not only may denature proteins, but may also induce reactions with the nearby nucleic acid and damage DNA.     

Low-energy (5-25 eV) electron damage to homo-oligonucleotides

Radiation-induced damage to homo-oligonucleotides is investigated by electron-stimulated desorption of neutral fragments from chemisorbed organic films. Six and 12 mers of cytidine phosphate (poly dCs) and thymidine phosphate (poly dTs) are chemisorbed from various solutions onto a crystalline gold substrate by a thiol modification at the 3' end and are irradiated under ultra-high vacuum conditions with 5-25 eV electrons. The mass selected neutral desorption yields consist mainly of fragments of the DNA bases, i.e. CN and OCN (and/or H2NCN for poly dCs) from both poly dCs and poly dTs, indicating that the electrons interact specifically via fragmentation of the aromatic ring of either of the bases. Other heavier fragments are also detected such as H3CC-CO from poly dTs. The yields generally possess a threshold near 5 eV and a broad maximum around 12-13 eV incident electron energy. Dissociative electron attachment as well as electronically excited neutral or cation states are believed to be responsible for the various desorption yields. The latter yields are consistently larger for oligos chemisorbed from water and acetone solutions, compared to methanol solution. The invariance of the fragment yield intensities with oligo length suggests that the molecules are likely to adsorb almost parallel to the surface.     

Effective cross sections for production of single-strand breaks in plasmid DNA by 0.1 to 4.7 eV electrons

We determined effective cross sections for production of single-strand breaks (SSBs) in plasmid DNA [pGEM 3Zf(-)] by electrons of 10 eV and energies between 0.1 and 4.7 eV. After purification and lyophilization on a chemically clean tantalum foil, dry plasmid DNA samples were transferred into a high-vacuum chamber and bombarded by a monoenergetic electron beam. The amount of the circular relaxed DNA in the samples was separated from undamaged molecules and quantified using agarose gel electrophoresis. The effective cross sections were derived from the slope of the yield as a function of exposure and had values in the range of 10(-15)- 10(-14) cm2, giving an effective cross section of the order of 10(-18) cm2 per nucleotide. Their strong variation with incident electron energy and the resonant enhancement at 1 eV suggest that considerable damage is inflicted by very low-energy electrons to DNA, and it indicates the important role of pi* shape resonances in the bond-breaking process. Furthermore, the fact that the energy threshold for SSB production is practically zero implies that the sensitivity of DNA to electron impact is universal and is not limited to any particular energy range.     

Alteration of protein constituents induced by low-energy (<35 eV) electrons: II. Dissociative electron attachment to amino acids containing cyclic groups

We report measurements of the desorption of anions from thin condensed films of tryptophan (Trp), histidine (His) and proline (Pro) stimulated by 5-35 eV electron impact. H-, O-, OH- and CN- desorb from Trp, His and Pro, whereas CH2- is observed only from Pro fragmentation. Below 12 eV, the anion yield functions exhibit resonant structures indicative of dissociative electron attachment. For all three amino acids, this process is likely to be initiated by the resonant capture of the incident electron at the NH3(+)-CH-.....-COO- and/or NH2-CH-.....-COOH group of the molecule. Temporary electron attachment to the ring leads to anion desorption only for tryptophan and proline. The energy-averaged yields measured at the detector of the mass spectrometer are (4.9, 0.3 and 54.0) x 10(-8) H-/incident electron and (3.4, 2.9, 1.8) x 10(-11) O-/incident electron, respectively, from Trp, His and Pro dissociation. Fragmentation of amino acids is found to be as intense as that of the nucleic acid bases. These results are discussed within the context of radiobiological damage induced by secondary electrons.     

Cross sections for low-energy (10-50 eV) electron damage to DNA

We report direct measurements of the formation of single-, double- and multiple strand breaks in pure plasmid DNA as a function of exposure to 10-50 eV electrons. The effective cross sections to produce these different types of DNA strand breaks were determined and were found to range from approximately 10(-17) to 3 x 10(-15) cm(2). The total effective cross section and the effective range for destruction of supercoiled DNA extend from 3.4 to 4.4 x 10(-15) cm(2) and 12 to 14 nm, respectively, over the range 10-50 eV. The variation of the effective cross sections with electron energy is discussed in terms of the electron's inelastic mean free path, penetration depth, and dissociation mechanisms, including resonant electron capture; the latter is found to dominate the effective cross sections for single- and double-strand breaks at 10 eV. The most striking observations are that (1) supercoiled DNA is approximately one order of magnitude more sensitive to the formation of double-strand breaks by low-energy electrons than is relaxed circular DNA, and (2) the dependence of the effective cross sections on the incident electron energy is unrelated to the corresponding ionization cross sections. This finding suggests that the traditional notion that radiobiological damage is related to the number of ionization events would not apply at very low energies.     

Ion desorption from DNA components irradiated with 0.5 keV ultrasoft X-ray photons

Positive ion desorption from thin films of DNA components, 2-deoxy-d-ribose, thymine, thymidine (dThd), and thymidine 5'-monophosphate (dTMP) was investigated in the oxygen K- shell edge excitation region using synchrotron ultrasoft X rays (538 eV). A large number of molecular fragments, H(+), CH(x)(+), C(2)H(x)(+), CO(+), CH(x)O(+), C(3)H(x)(+), C(2)H(x)O(+) and C(3)H(x)O(+) (x = 1, 2 and 3), were observed as desorbed ions from 2-deoxy-d-ribose. Some of these ions are related to simultaneous bond scission at particular C-C and C-O (or C-C) bonds in the furanose ring structure in the 2-deoxy-d-ribose molecule, indicating that the impact of photons on the oxygen atom and the impact of ejected secondary electrons (e.g. Auger electrons) cause an intense destruction of the furanose ring structure. In thymine thin films, H(+), CH(x)(+), CO(+), CH(x)O(+), C(2)H(x)N(+) and CH(x)NO(+) (x = 1, 2 and 3) fragments were observed. The yields of these ions were smaller than the yields from 2-deoxy-d-ribose. The desorption of CH(3)(+) from thymine might induce a molecular conversion from thymine to uracil. The mass patterns of dThd and dTMP, and especially that of dTMP, were similar to that of 2-deoxy-d-ribose, indicating that a number of ions were generated at the sugar site, even in the nucleotide molecule. It is therefore predicted that the sugar moiety is more fragile than the thymine base.     

Energy spectra of secondary electrons in water vapour

The purpose of this work is to present a method for the calculation of secondary electron spectra generated by photons in water vapour in the energy region from 10 eV to 10 MeV. The cross sections below and above 1 keV have been treated separately. Examples are given for secondary electron spectra for low-energy photons, <100 eV, in which all electrons are photoelectrons, and at higher energy regions, such as for ⁶⁰Co photons. The spectrum of the first generation of secondary electrons, produced by ⁶⁰Co photons, which are mainly due to incoherent scattering, was fitted with a set of polynomial functions which can be used as input for electron radiation action calculations.     

Fast atom bombardment combined with tandem mass spectrometry for determining structures of small oligonucleotides

A study of small (n = 3 to 6) oligonucleotide and the metastable and collisionally activated decompositions of their (M-H)- species desorbed by using fast atom bombardment (FAB) is reported. Data were obtained for both ribo- and 2'-deoxyribotrinucleotides and for 2'-deoxyribotetra-, penta-, and hexanucleotides. The favored metastable decompositions of all of the oligonucleotides studied are eliminations of neutral CONH and loss of BH, where B is the base moiety. The BH elimination, however, provides little sequence information in the higher oligonucleotides and the process is more indicative of the different bases present in the oligomer. The chemistry observed upon collisional activation changes as one goes from trinucleotides to hexanucleotides. The formation of sequence ions is more facile for processes involving the 3' terminus, allowing the sequence to be determined. As one goes to the higher oligonucleotides, however, several different competitive fragmentation processes become as facile as or more facile than the reactions giving the sequence ions. This hinders proper ion assignments and makes sequence determination difficult.     

Laser-generated plasmas at INFN-LNS

Hot plasmas can be generated by fast and intense laser pulses ablating solids placed in vacuum. A Nd:Yag laser operating at the fundamental and second harmonics with 9-ns pulses (maximum energy of 900 mJ) focused on metallic surfaces produces high ablation yields of the order of μg/pulse and dense plasma that expands adiabatically at supersonic velocity along the normal to the target surface. The plasma emits neutral and charged particles. Charge states up to 10+ have been measured in heavy elements ablated with intensities of the order of 10¹⁰ W/cm². The ion temperature of the plasma is evaluated from the ion energy distributions measured with an ion energy analyzer. The electron temperature is measured through Faraday cups placed at the end of long drift tubes by using time-of-flight technique. The neutral temperature is measured with a special mass quadrupole spectrometer placed along the normal to the target surface. The plasma temperature increases with the laser pulse intensity. The ion temperature reaches values of the order of 400 eV, the electron temperature is of the order of 1 keV for hot electrons and 0.1 eV for thermal electrons, and the neutral temperature is of the order of 200 eV. The experimental apparatus, the diagnostic techniques, and the procedures for the plasma temperature characterization will be presented and discussed in detail. Published in Russian in Fizika Plazmy, 2006, Vol. 32, No. 6, pp. 558–564. The text was submitted by the authors in English.     

Absolute and effective cross-sections for low-energy electron-scattering processes within condensed matter

Within the last two decades, a number of experimental techniques have been developed to measure mean free paths and absolute and effective cross-sections for various processes related to the interaction of low-energy electrons with condensed matter. In all of the experiments, a monochromatic electron beam impinges on a thin multilayer film composed of atoms and/or molecules condensed on a metal or semiconductor substrate held at cryogenic temperatures in an ultra-high-vacuum system. Depending on the apparatus, cross-sections are obtained from low-energy electron transmission (LEET), high-resolution electron energy loss (HREEL), x-ray photoelectron (XPS) spectroscopy, electron-stimulated desorption (ESD) of neutral and ions, or a combination of these techniques. Quasi-elastic and inelastic mean free paths have been extracted from LEET data. This method has also served to generate absolute cross-sections for electron trapping and fragment production from the dissociation of transient molecular anions. In amorphous ice, a complete set of absolute cross-sections for all inelastic losses by 1–20 eV electrons has been obtained from HREEL data. Effective cross-sections for neutral and ionic radical formation were generated by desorption and XPS experiments. These various methods are briefly described in this article, and the corresponding cross-sections in the range 0–20 eV summarized. Received: 10 September 1998 / Accepted: 22 October 1998     

Monte Carlo simulation of strand-break induction on plasmid DNA in aqueous solution by monoenergetic electrons

The radiation-induced process of strand breaks on pBR322 plasmid DNA in aqueous solution for different energy electrons was studied by Monte Carlo simulation. Assumptions of induction mechanisms of single- and double-strand breaks (SSBs and DSBs) used in the simulation are that SSB is induced by OH or H reaction with DNA and that DSB is induced by two SSBs on the opposite strands within 10 bp. Dose-response relationships of SSBs and DSBs were demonstrated for monoenergetic electrons of 100 eV, 10 keV, 1 keV and 1 MeV, and the yields of SSB and DSB were calculated. The dose-response relationships of SSBs and DSBs can be fitted by linear and linear-quadratic functions, respectively. The ratio of quadratic to linear components of DSB induction changes due to the electron energy. A high contribution of the linear component is observed for 1 keV electrons in the dose range below 160 Gy. The yields of SSBs and DSBs for all examined electron energies lie well within the experimental data when the probability of strand-break induction by OH and H is assumed to be around 0.1-0.2. The yield of SSBs has a minimum at 1 keV, while the yield of DSBs has a maximum at 1 keV in the examined energies. The strand breaks are formed most densely for 1 keV electrons.     

Effect of low-energy electron irradiation on DNA damage by Fe3+ ion

We investigated the combined effects of low-energy electron irradiation and Fe(3+) ion on DNA damage. We used lyophilized pBR322 plasmid DNA films with various concentrations (0 ~ 7 mM) of Fe(3+) ions and irradiation with monochromatic, low-energy 3 or 5 eV electrons for these studies. DNA-Fe(3+) films were recovered and analyzed by agarose gel electrophoresis to identify and compare the effects of Fe(3+) ions and/or low-energy electrons alone or in combination on DNA damage. In nonirradiated DNA-Fe(3+) films, there was little DNA damage observed (less than 10% of the total DNA loaded on the gel appeared damaged) for Fe(3+) ion up to 7 mM concentration. In irradiated DNA films without Fe(3+) ions, there was also very little DNA damage observed (less than 3% of the total DNA loaded on the gel appeared damaged). However, when DNA-Fe(3+) films, were irradiated with low-energy electrons, DNA damage was significantly increased compared to the sum of the damage caused both by either Fe(3+) ion or low-energy electrons irradiation alone. We proposed that both DEA and/or electron transfer processes might play a role in the enhanced DNA damage when DNA-Fe(3+) films were irradiated by low-energy electrons.     

Base release in nucleosides induced by low-energy electrons: a DFT study

Low-energy electrons are known to induce strand breaks and base damage in DNA and RNA through fragmentation of molecular bonding. Recently the glycosidic bond cleavage of nucleosides by low-energy electrons has been reported. These experimental results call for a theoretical investigation of the strength of the C(1)'-N link in nucleosides (dA, dC and dT) between the base and deoxyribose before and after electron attachment. Through density functional theory (DFT) calculations, we compare the C(1)'-N bond strength, i.e., the bond dissociation energy of the neutral and its anionic radical, and find that an excess electron effectively weakens the C(1)'- N bond strength in nucleosides by 61-75 kcal/mol in the gas phase and 76-83 kcal/mol in the solvated environment. As a result, electron-induced fragmentation of the C(1)'-N bond in the gas phase is exergonic for dA (DeltaG=-14 kcal/mol) and for dT (DeltaG=-6 kcal/mol) and is endergonic (DeltaG=+1 kcal/ mol) only for dC. In the gas phase all the anionic nucleosides are found to be in valence states. Solvation is found to increase the exergonic nature by an additional 20 kcal, making the fragmentation both exothermic and exergonic for all nucleoside anion radicals. Thus C(1)'-N bond breaking in nucleoside anion radicals is found to be thermodynamically favorable both in the gas phase and under solvation. The activation barrier for the C(1)'-N bond breaking process was found to be about 20 kcal/mol in every case examined, suggesting that a 1 eV electron would induce spontaneous cleavage of the bond and that stabilized anion radicals on the DNA strand would undergo base release at only a modest rate at room temperature. These results suggest that base release from nucleosides and DNA is an expected consequence of low-energy electron-induced damage but that the high barrier would inhibit this process in the stable anion radicals.     

Energy loss of hydrogen- and helium-ion beams in DNA: calculations based on a realistic energy-loss function of the target

We have calculated the electronic energy loss of proton and α-particle beams in dry DNA using the dielectric formalism. The electronic response of DNA is described by the MELF-GOS model, in which the outer electron excitations of the target are accounted for by a linear combination of Mermin-type energy-loss functions that accurately matches the available experimental data for DNA obtained from optical measurements, whereas the inner-shell electron excitations are modeled by the generalized oscillator strengths of the constituent atoms. Using this procedure we have calculated the stopping power and the energy-loss straggling of DNA for hydrogen- and helium-ion beams at incident energies ranging from 10 keV/nucleon to 10 MeV/nucleon. The mean excitation energy of dry DNA is found to be I = 81.5 eV. Our present results are compared with available calculations for liquid water showing noticeable differences between these important biological materials. We have also evaluated the electron excitation probability of DNA as a function of the transferred energy by the swift projectile as well as the average energy of the target electronic excitations as a function of the projectile energy. Our results show that projectiles with energy ?100 keV/nucleon (i.e., around the stopping-power maximum) are more suitable for producing low-energy secondary electrons in DNA, which could be very effective for the biological damage of malignant cells.     

Radiosensitization of DNA by gold nanoparticles irradiated with high-energy electrons

Zheng, Y., Hunting, D. J., Ayotte, P. and Sanche, L. Radiosensitization of DNA by Gold Nanoparticles Irradiated with High-Energy Electrons. Radiat. Res. 168, 19-27 (2008). Thin films of pGEM-3Zf(-) plasmid DNA were bombarded by 60 keV electrons with and without gold nanoparticles. DNA single- and double-strand breaks (SSBs and DSBs) were measured by agarose gel electrophoresis. From transmission electron micrographs, the gold nanoparticles were found to be closely linked to DNA scaffolds, probably as a result of electrostatic binding. The probabilities for formation of SSBs and DSBs from exposure of 1:1 and 2:1 gold nanoparticle:plasmid mixtures to fast electrons increase by a factor of about 2.5 compared to neat DNA samples. For monolayer DNA adsorbed on a thick gold substrate, the damage increases by an order of magnitude. The results suggest that the enhancement of radiosensitivity is due to the production of additional low-energy secondary electrons caused by the increased absorption of ionizing radiation energy by the metal, in the form of gold nanoparticles or of a thick gold substrate. Since short-range low-energy secondary electrons are produced in large amounts by any type of ionizing radiation, and since on average only one gold nanoparticle per DNA molecule is needed to increase damage considerably, targeting the DNA of cancer cells with gold nanoparticles may offer a novel approach that is generally applicable to radiotherapy treatments.     

Comparative microdosimetry of photoelectrons and Compton electrons: an analysis in terms of generalized proximity functions

A current discussion on mammography screening is focused on claims of high relative biological effectiveness (RBE) of mammography X rays compared to conventional 200 kV X rays. An earlier assessment in terms of the electron spectra of these radiations has led to the conclusion that the RBE is bound to be less than 2, regardless of specific model assumptions and the microdosimetric properties of electrons. The present study extends this result in terms of the microdosimetric proximity function, t(x), for electrons, which is essentially the spatial auto-correlation function of energy within particle tracks. If pairs of DNA lesions, e.g. chromosome breaks or deletions, bring about the observed damage, the value t(x) determines for a specified radiation the relative frequency of pairs of lesions a distance x apart. The effectiveness of the radiation is thus proportional to an average of the values of t(x) over the distances, x, for which lesions can combine. The analysis suggests that 15 keV electrons can have a low-dose relative biological effectiveness (RBE(M)) of 1.6 relative to 40 keV electrons if the interaction distances do not exceed about 1 micro m. An extension of the concept, the reduced proximity function, t(delta)(x), permits the inclusion of models with an energy threshold, such as delta = 100 eV, 500 eV or 2 keV, for the formation of each of the DNA lesions. This makes it possible to assess the potential impact of the Auger electrons which accompany most photoelectrons, but only a minority of the Compton electrons. It is found that the Auger electrons could make photoelectrons substantially more effective than Compton electrons at energies below 10 keV but not at energies above 15 keV. The conclusions obtained for the RBE of 15 keV electrons relative to 40 keV electrons will be roughly representative of the RBE of mammography X rays relative to conventional 200 kV X rays.     

On (3)H beta-particle and (60)Co gamma irradiation of aqueous systems

The chemistry of water and aqueous solutions is very different after irradiation with (3)H beta particles and high-energy electrons or (60)Co gamma rays. The greater the linear energy transfer (LET) of the medium for (3)H beta particles compared to high-energy electrons or (60)Co gamma rays leads to an increased local concentration of reactants. There is an increased amount of intratrack chemistry, which reduces the escape yield of and OH by about 50%, but increases the yield of H(2) by about 50% and of H(2)O(2) by about 35%. Analysis of stochastic-diffusion kinetic calculations employing simulated track structures reveals that the yield of H(2) produced by diffusion-kinetic processes increases significantly for (3)H beta particles compared to (60)Co gamma radiation, while production of H(2) by sub-picosecond processes is essentially the same. In both (3)H beta-particle and (60)Co gamma radiolysis, the reactions + and are equally important in the production of H(2). In the former case, each reaction has a yield of approximately 0.18, and in the latter a yield of approximately 0.08. In neutral water, the reaction (H + H) is negligible. The yield of Fe(III) in (3)H beta-particle radiolysis of the Fricke dosimeter is much smaller than in radiolysis with more energetic electrons. Simulations show that this change is primarily due to the reduced escape yield of H, formed from the scavenging of by the bulk H(3)O(+) of the acid. The chemical differences observed in experiments, and in calculations, reflect the underlying structure of the electron tracks: Examination of the track structure simulations demonstrates that primary events are considerably more well-separated in high-energy electron tracks compared to (3)H beta-particle tracks.     

Electron and photon spectra for three gadolinium-based cancer therapy approaches

Some recent neutron capture therapy research has focused on using compounds containing the element gadolinium, which produces internal conversion and Auger cascade electrons. The low-energy, short-range Auger electrons are absorbed locally and increase cell killing dramatically as the gadolinium compounds are introduced into the cell nucleus and bind to the DNA. Detailed electron and photon spectra are needed for biophysical modeling and Monte Carlo calculations of damage to DNA. This paper presents calculated electron and photon spectra for three cases: thermal neutron absorption by (157)Gd, the beta-particle decay of (159)Gd, and the K-shell photoelectric event in gadolinium. The Monte Carlo sampling of atomic and nuclear transitions for each of the three cases was used to calculate a large number of representative decays. The sampled decays were used to determine average emissions and energy deposited in small spheres of tissue. The kinetic energy nuclear recoil from gamma-ray and electron emissions was calculated and found to be more than 10 eV for 26% of all (157)Gd neutron capture reactions.