Low-dose Neutron Research Articles

Studies on proton irradiation-induced modifications of KU1 and KS-4V quartz glasses ultraviolet transmission properties

KU1 and KS-4V quartz glasses, known to be radiation-resistant insulator materials, are potential candidates for manufacturing diagnostics windows and optical fibres for ITER. KU1 and KS-4V quartz glass samples were irradiated with 12.6 and 14 MeV protons, respectively, at the Bucharest Tandem accelerator to simulate ionization and displacement damage and study the ultraviolet (UV) transmission properties modification. The proton irradiations produce absorption bands in KU1 at 215 and 265 nm, and in KS-4V at 215 and 245 nm, which saturate by about 10 MGy. This behaviour is very similar to results obtained for gamma, electron, and neutron irradiations, highlighting the importance of ionization, and indicating that proton irradiations may be used to simulate low dose neutron (⩽10 20 n/m 2) irradiations.

Microstructural investigation, using small-angle neutron scattering (SANS), of Optifer steel after low dose neutron irradiation and subsequent high temperature tempering

The microstructural effect of low dose neutron irradiation and subsequent high temperature tempering in the reduced activation ferritic/martensitic steel Optifer (9.3 Cr, 0.1 C, 0.50 Mn, 0.26 V, 0.96 W, 0.66 Ta, Fe bal wt%) has been studied using small-angle neutron scattering (SANS). The investigated Optifer samples had been neutron irradiated, at 250 °C, to dose levels of 0.8 dpa and 2.4 dpa. Some of them underwent 2 h tempering at 770 °C after the irradiation. The SANS measurements were carried out at the D22 instrument of the High Flux Reactor at the Institut Max von Laue – Paul Langevin, Grenoble, France. The differences observed in nuclear and magnetic SANS cross-sections after subtraction of the reference sample from the irradiated one suggest that the irradiation and the subsequent post-irradiation tempering produce the growth of non-magnetic defects, tentatively identified as microvoids.

Microstructural investigation of low-dose neutron irradiation effects in martensitic steels for nuclear application by means of small-angle neutron scattering

The microstructural effect of low-dose neutron irradiation and subsequent high-temperature tempering in the reduced activation ferritic/martensitic steel F82H-mod. (7.73 Cr, 0.09 C, 0.08 Mn, 0.19 V, 2.06 W, 0.02 Ta, wt%, bal. Fe) has been studied using small-angle neutron scattering (SANS). The investigated samples were irradiated with thermal neutrons at 523 K, to dose levels of 2.4 displacements per atom then tempered for 2 h at 1043 K. The SANS measurements were carried out at the D22 instrument of the High Flux Reactor at the Institut Max von Laue–Paul Langevin, Grenoble, France. The differences observed in nuclear and magnetic small-angle neutron scattering cross-sections after subtraction of the reference sample from the irradiated one suggest that the irradiation and the subsequent post-irradiation tempering produce the growth of non-magnetic precipitates; the results are also compared with those obtained on other ferritic/martensitic steels, with different chemical composition, irradiated under the same conditions.

06/02302 Thermal conductivity degradation of ceramic materials due to low temperature, low dose neutron irradiation: Snead, L. L. et al. Journal of Nuclear Materials, 2005, 340, (2–3), 187–202.

06/02302 Thermal conductivity degradation of ceramic materials due to low temperature, low dose neutron irradiation: Snead, L. L. et al. Journal of Nuclear Materials, 2005, 340, (2–3), 187–202.

Annealing behaviors of vacancy in varied neutron irradiated Czochralski silicon

The difference of annealing behaviors of vacancy-oxygen complex (VO) in varied dose neutron irradiated Czochralski silicon: (S1 5×10 17 n/cm 3 and S2 1.07×10 19 n/cm 3) were studied. The results show that the VO is one of the main defects formed in neutron irradiated Czochralski silicon (CZ-Si). In this defect, oxygen atom shares a vacancy, it is bonded to two silicon neighbors. Annealed at 200 °C, divacancies are trapped by interstitial oxygen(O i) to form V 2O (840 cm −1). With the decrease of the 829 cm −1 (VO) three infrared absorption bands at 825 cm −1 (V 2O 2), 834 cm −1 (V 2O 3) and 840 cm −1 (V 2O) will rise after annealed at temperature range of 200–500 °C. After annealed at 450–500 °C the main absorption bands in S1 sample are 834 cm −1, 825 cm −1 and 889 cm −1 (VO 2), in S2 is 825 cm −1. Annealing of A-center in varied neutron irradiated CZ-Si is suggested to consist of two processes. The first is due to trapping of VO by O i in low dose neutron irradiated CZ-Si (S1) and the second is due to capture the wandering vacancy by VO, etc, in high dose neutron irradiated CZ-Si (S2), the VO 2 plays an important role in the annealing of A-center. With the increase of the irradiation dose, the annealing behavior of A-center is changed.

Dose-Response and Large Relative Biological Effectiveness of Fast Neutrons with Regard to Mouse Fetal Cerebral Neuron Apoptosis

To evaluate the relative biological effectiveness (RBE) of low doses of neutrons on fetal nervous development, [C57BL/6J x C3H/He] hybrid (B6C3F1) mice were exposed to cyclotron-derived fast neutrons with peak energy of 10 MeV (0.02-1.0 Gy) or 137Cs-generated gamma-rays (0.1-2.0 Gy) on embryonic day 13.5. We then evaluated the incidence of neuronal apoptosis in the cerebral cortex 24 hours after irradiation. Neuronal apoptosis increased in a dose-dependent manner in both neutron- and gamma-ray-irradiated groups: even at the lowest dose, a minimal increase in the apoptotic index was noted in response to both types of radiation. The dose-response curves were best fitted to linear quadratic models, and the evaluated RBE was 9.8, which was considered to be large for a prenatal effect and acute tissue injury induced by a low dose of neutrons.

Radiobiologie avec des neutrons rapides

Radiobiology with fast neutrons. Human melanoma cells particularly resistant to high energy X rays were irradiated with 14 MeV neutrons at low doses ranging from 5 cGy to 1.12 Gy and using two different dose rates: a very low one (0.8 mGy mn -1 ) and a moderate one (40 mGy mn -1 ). The biological effects of neutrons were studied both by cell survival assay and chromosomal aberrations analysis. Unusual features of the survival curve at low dose rate were observed as a marked degree of cell killing at 5 cGy followed by a plateau of survival from 10 to 32.5 cGy. At low dose rate, the levels of induced chromosomal aberrations tended also to reach a plateau with doses ranging from 10 to 30 cGy. Both phenomena disappear at higher dose rate. The existence of a plateau observed after low-dose neutrons suggests that a repair process might be induced after a threshold dose of 5-7.5 cGy with a saturation around 32.5 cGy. These findings may be of interest for the domain of neutrons radiobiology and radioprotection.

Evaluation of gamma and neutron irradiation effects on the properties of mica film capacitors

We present an investigation of gamma and neutron radiation effects on mica film capacitors from an electrical point of view. We have studied quantitatively the effects of gamma and neutron irradiation on mica film capacitors of thickness, 20 and 40 μm (0.7874 and 1.5748 mil) with two different areas, 01 and 04 cm2. The capacitance has been measured at room temperature in the frequency range 100 Hz-10 MHz. Negligible change in the capacitance due to high gamma dose of60Co, 15 kGy at dose rate 0.25 kGy/h, has been observed. However, appreciable change in the capacitance has been observed due to low doses of fast neutrons (cumulative dose, 115 cGy) with flux ∼ 9.925 x 107 neutrons/cm2 h from252Cf neutron source of fluence, 2.5 × 107 neutrons/s. We have also observed that the impact of gamma and neutron irradiation is more at frequencies higher than 10 kHz. These results show that the mica capacitors do not show any radiation response below 10 kHz. The study shows the radiation response of mica film capacitors to gamma and fast neutron radiations. Mica capacitors show low gamma radiation response in comparison to fast neutron radiation, because a total dose of kGy order has been given by gamma source and only few cGy dose has been given by fast neutron source.

Thermal conductivity degradation of ceramic materials due to low temperature, low dose neutron irradiation

The thermal conductivity degradation due to low-temperature neutron irradiation is studied and quantified in terms of thermal resistance terms. Neutron irradiation is assumed to have no effect on umklapp scattering. A theoretical model is presented to quantify the relative phonon-scattering effectiveness of the three dominant defect types produced by neutron irradiation: point defects, dislocation loops and voids. Several commercial ceramics have been irradiated with fission reactor fast neutrons at low temperatures to produce defects. Materials include silicon carbide, sapphire, polycrystalline alumina, aluminum nitride, silicon nitride, beryllium oxide, and a carbon fiber composite. The neutron dose corresponded to 0.001 and 0.01 displacements per atom (dpa) for a ∼60°C irradiation and 0.01 and 0.1dpa for a ∼300°C irradiation. Substantial thermal conductivity degradation occurred in all of the materials except BeO following irradiation at 60°C to a dose of only 0.001dpa. The data are discussed in terms of the effective increase in thermal resistance caused by the different irradiation conditions. Evidence for significant point defect mobility during irradiation at 60 and 300°C was obtained for all of the ceramics. The thermal stability of the radiation defects was investigated by isochronal annealing up to 1050°C.

Cancer risk estimates for gamma-rays with regard to organ-specific doses

Part I of this study presented an analysis of the solid cancer mortality data for 1950-1997 from the Japanese life-span study of the A-bomb survivors to assess the cancer risk for gamma-rays in terms of the organ-specific dose for all solid cancers combined. Compared to earlier analyses, considerably more curvature in the dose-effect relation is indicated by these computations, which now suggests a dose and dose-rate effectiveness factor of about 2. The computations are extended here in order to explore the site-specific solid cancer risks for various organs. A computational method has been developed whereby the site-specific cancer risks are all simultaneously computed with global age and gender effect modifiers. This provides a more parsimonious representation with fewer parameters and avoids the large relative standard errors which would otherwise result. The sensitivity of site-specific risks to the choices of the neutron RBE is examined. The site-specific risk estimates are quite sensitive to the neutron RBE for the least shielded organs such as the breast, bladder and oesophagus. For the deeper lying organs, such as the gallbladder, pancreas and uterus, the impact of the neutrons is much lower. With an assumed neutron RBE of 35, which is in line with results on low neutron doses in major past studies on rodents and which corresponds approximately to the current ICRP radiation weighting factor for neutrons, the neutrons appear to contribute about 40% of the observed excess cancer risk in the breast, i.e. the organ that is closest to the body surface. However, this neutron contribution fraction is only about 10% for deeper lying organs, such as the colon.

Materials design data for reduced activation martensitic steel type EUROFER

Materials design limits derived so far from the data generated in Europe for the reduced activation ferritic/martensitic (RAFM) steel type Eurofer are presented. These data address the short-term needs of the ITER Test Blanket Modules and a DEMOnstration fusion reactor. Products tested include plates, bars, tubes, TIG and EB welds, as well as powder consolidated blocks and solid–solid HIP joints. Effects of thermal ageing and low dose neutron irradiation are also included. Results are sorted and screened according to design code requirements before being introduced in reference databases. From the physical properties databases, variations of magnetic properties, modulus of elasticity, density, thermal conductivity, thermal diffusivity, specific heat, mean and instantaneous linear coefficients of thermal expansion versus temperature are derived. From the tensile and creep properties databases design allowable stresses are derived. From the instrumented Charpy impact and fracture toughness databases, ductile to brittle transition temperature, toughness and behavior of materials in different fracture modes are evaluated. From the fatigue database, total strain range versus number of cycles to failure curves are plotted and used to derive fatigue design curves. Cyclic curves are also derived and compared with monotonic hardening curves. Finally, irradiated and aged materials data are compared to ensure that the safety margins incorporated in unirradiated design limits are not exceeded.

Role of Radiation-Induced Grain Boundary Segregation in Irradiation Assisted Stress Corrosion Cracking

Isolation of microstructural and microchemical effects on irradiation assisted stress corrosion cracking (IASCC) was attempted by means of low-dose high-temperature neutron irradiation in a material test reactor to get better understanding on IASCC. Microstructure, grain boundary segregation, hardness and SCC susceptibility were examined on stainless steels irradiated to 0.8 dpa at around 673 K. The irradiation caused well-developed grain boundary segregation without notable hardening or microstructural changes. No IASCC was found in 593 K hydrogenated water whereas the steels were highly susceptible to IASCC in 561 K oxygenated water. The results suggest that grain boundary segregation, probably Cr depletion, is sufficient to cause IASCC in oxygenated water and that other radiation-induced changes such as microstructure and hardening are required for IASCC in hydrogenated water.

Low-temperature low-dose neutron irradiation effects on beryllium

Mechanical property results are presented for high quality beryllium materials subjected to low-temperature, low-dose neutron irradiation in water-moderated reactors. Materials chosen were the S-65C ITER candidate material produced by Brush Wellman, Kawecki Berylco Industries P0 beryllium, and a high-purity zone refined beryllium. Mini-sheet tensile and thermal diffusivity specimens were irradiated in the temperature range of ∼100–300 °C with a fast ( E>0.1 MeV) neutron fluence of 0.05–1.0 × 10 25 n/m 2 in the high flux isotope reactor (HFIR) at the Oak Ridge National Laboratory and the high flux beam reactor (HFBR) at the Brookhaven National Laboratory. As expected from earlier work on beryllium, both materials underwent significant tensile embrittlement with corresponding reduction in ductility and increased strength. Both thermal diffusivity and volumetric expansion were measured and found to have negligible changes in this temperature and fluence range. Of significance from this work is that while both materials rapidly embrittle at these ITER relevant irradiation conditions, some ductility (1–2%) remains, which contrasts with a body of earlier work including recent work on the Brush-Wellman S-65C material irradiated to slightly higher neutron fluence.

Effect of Low Neutron Doses on the Properties of Synthetic Diamond Crystals

The effect of neutron irradiation on the properties of diamond crystals prepared in the Mn–Ni–C system (5.5 GPa, 1620–1670 K, 120–300 s) was studied. The results indicate that irradiation increases the strength of the crystals and reduces the concentration of paramagnetic centers. The observed changes are interpreted in terms of interaction between intrinsic defects and impurities.

Protein synthesis, cellular defence and hprt-mutations induced by low-dose neutron irradiation

The mutagenic potential of low-dose neutrons was studied to demonstrate RAR, and was aimed to recognise the correlation of the effectiveness of antioxidant enzymes and the synthesis of certain proteins. The CHO cells were adapted by priming exposure with neutron doses of 0.5–50 mGy and with dose rates of 1.59–10 mGy min −1. A total of 2–4 Gy of gamma radiation after 1–48 h then challenged the cells. The number of induced hprt-mutants scored was to be reduced by 33–57% when priming was administered, compared to a single dose of challenge, depending on the adapting dose and dose rate. For adaptation, the most appropriate priming dose was 2 mGy and 5 h was the optimal time for its development. Six main, induced proteins were detected (109, 70, 65, 60, 57, 45 kDa). The protein 70 kDa was identified after the priming and challenge dose, but it disappeared after their combination. The capacity of the endogenous antioxidant (superoxide dismutase, SOD) to remove reactive oxygen species had been shown to be enhanced. It could be concluded that low-dose neutrons also induced RAR and the contribution of induced proteins; furthermore, the antioxidant potential would be in part responsible for the RAR.

Risk estimates for fast neutrons and implications for radiation protection

The risks of low neutron doses are of increasing interest with regard to exposures in aviation, in the transport of nuclear fuel, or even in an accident as that in Tokaimura. In the absence of epidemiological information, the neutron risk coefficient is currently taken to be the product of two uncertain low-dose extrapolations, nominal risk coefficient for the photons and presumed maximum relative biological effectiveness of the neutrons. An approach is presented here that avoids the low-dose extrapolations and invokes, instead, the product of the excess risk from epidemiological observations at an intermediate reference dose of γ-rays—here taken to be 1 Gy—and the assumed value, R 1, of the neutron RBE against this reference dose. With R 1 between 20 and 50 the solid, the cancer mortality data of the A-bomb survivors provide an excess relative risk (ERR) for the neutrons between 8 and 16/Gy. This result is converted into a risk coefficient in terms of the lifetime attributable risk (LAR) relative to the neutron effective dose as defined by ICRP. The result is at high neutron energies somewhat in excess of the current ICRP nominal risk factor, while it agrees with ICRP for the degraded fast neutron spectra that are of major pragmatic interest. The so-called Hiroshima neutron discrepancy will not influence the neutron risk estimate because any remaining uncertainty of the neutron doses in Hiroshima is minor at the reference dose of 1 Gy.

Feasibility of assessing the carcinogenicity of neutrons among neutron therapy patients.

Nuclear workers, oil well loggers, astronauts, air flight crews, and frequent fliers can be exposed to low doses of neutrons, but the long-term human health consequences of neutron exposure are unknown. While few of these exposed populations are suitable for studying the effects of neutron exposure, patients treated with neutron-beam therapy might be a source of information. To assess the feasibility of conducting a multi-center international study of the late effects of neutron therapy, we surveyed 23 cancer centers that had used neutron beam therapy. For the 17 responding institutions, only 25% of the patients treated with neutrons (2,855 of 11,191) were alive more than 2 years after treatment. In a two-center U.S. pilot study of 484 neutron-treated cancer patients, we assessed the feasibility of obtaining radiotherapy records, cancer incidence and other follow-up data, and of estimating patient organ doses. Patients were treated with 42 MeV neutrons between 1972 and 1989. Applying a clinical equivalence factor of 3.2 for neutrons, total average organ doses outside the treatment beam ranged from 0.14 to 0.29 Gy for thyroid, 0.40 to 2.50 Gy for breast, 0.63 to 2.35 Gy for kidney, and 1.12 to 1.76 Gy for active bone marrow depending upon the primary cancer treatment site. We successfully traced 97% of the patients, but we found that patient survival was poor and that chemotherapy was not confirmable in a quarter of the patients. Based on our findings from the international survey and the feasibility study, we conclude that a large investigation could detect a fivefold or higher leukemia risk, but would be inadequate to evaluate the risk of solid cancers with long latent periods and therefore would likely not be informative with respect to neutron-related cancer risk in humans.

Irradiation facility for boron neutron capture therapy application based on a rf-driven D–T neutron source and a new beam shaping assembly (abstract)

Selecting the best neutron source for boron neutron capture therapy (BNCT) requires optimizing neutron beam parameters. This involves solving many complex problems. Safety issues related to the use of nuclear reactor in hospital environments, as well as lower costs have led to interest in the development of accelerator-driven neutron sources. The BNCT research programs at the Nuclear Departments of Pisa and Genova Universities (DIMNP and DITEC) focus on studies of new concepts for accelerator-based DT neutron sources. Simple and compact accelerator designs using relatively low deuteron beam energy, ∼100 keV, have been developed which, in turn, can generate high neutron yields. New studies have been started for optimization of moderator materials for the 14.1 MeV DT neutrons. Our aim is to obtain an epithermal neutron beam for therapeutic application at the exit end, with minimal beam intensity losses, the specific goal is to achieve an epithermal neutron flux of at least of 1×109 n/cm2 s at the beam port, with low gamma and fast neutron dose contamination. According to the most recent neutron BNCT beam parameters some moderating and spectrum shifter materials and geometrical configurations have thus far been tested, and neutron and gamma beam data at beam port have been computed. A possible beam shaping assembly model has been designed. This research demonstrates that a DT neutron source could be successfully implemented for BNCT application, with performance surpassing the minimum requirements stated above, using DT neutron sources with yields in the range 1013–1014 n/s. The latest Monte Carlo simulation results of an accelerator based facility which relies on a rf-driven DT fusion neutron generator will be presented.

Effect of boron neutron capture therapy for melanotic and amelanotic melanoma transplanted into mouse brain.

In order to develop a protocol to treat brain metastatic melanoma using our 10B-p-boronophenylalanine (BPA) boron neutron capture therapy (BNCT), we initiated the following studies (i), Comparative analyses of boron biodistribution between melanoma proliferating in the brain and skin among melanotic and amelanotic types, and (ii) Therapeutic evaluation of BPA-BNCT for brain melanoma models of both types, using survival times. Our present data have revealed that boron concentration in melanoma proliferating in the brain, the major prerequisite for successful BNCT, showed a positive correlation to melanin synthesizing activity in the same way as melanoma proliferating in skin. Further, the boron concentration ratio of melanoma to normal surrounding tissue for brain melanoma models was considerably higher than that for subcutaneous (s.c.) ones because of the existence of the blood-brain barrier (BBB). Additionally, from analyses of median and mean survival times following BNCT using low, middle, and high neutron doses, the therapeutic effect of BNCT for the amelanotic A1059 melanoma appeared at first glance to be higher than that for the highly BPA attracting and highly relative biological effect equivalent dose obtaining B15b melanoma. As the survival time was dependent on both regression and regrowth curves, and because the brain melanoma model in small animals made it difficult to evaluate these curves separately, we further examined the in vivo growth curve of both types of melanomas following implantation in s.c. tissue. The melanotic B15b melanoma was indeed found to possess much higher growth rate as compared with that of the amelanotic A1059 melanoma. The significance of boron biodistribution studies and BNCT survival curve analyses in forming an effective clinical protocol for individual human cases of melanoma brain metastasis is discussed.

Effects of low-dose neutrons applied at reduced dose rate on human melanoma cells.

Human melanoma cells that are resistant to gamma rays were irradiated with 14 MeV neutrons given at low doses ranging from 5 cGy to 1.12 Gy at a very low dose rate of 0.8 mGy min(-1) or a moderate dose rate of 40 mGy min(-1). The biological effects of neutrons were studied by two different methods: a cell survival assay after a 14-day incubation and an analysis of chromosomal aberrations in metaphases collected 20 h after irradiation. Unusual features of the survival curve at very low dose rate were a marked increase in cell killing at 5 cGy followed by a plateau for survival from 10 to 32.5 cGy. The levels of induced chromosomal aberrations showed a similar increase for both dose rates at 7.5 cGy and the existence of a plateau at the very low dose rate from 15 to 30 cGy. The existence of a plateau suggests that a repair process after low-dose neutrons might be induced after a threshold dose of 5-7.5 cGy which compensates for induced damage from doses as high as 32.5 cGy. These findings may be of interest for understanding the relative biological effectiveness of neutrons and the effects of environmental low-dose irradiation.