Search for bosonic superweakly interacting massive dark matter particles with the XMASS-I detector.

Bosonic superweakly interacting massive particles (super-WIMPs) are a candidate for warm dark matter. With the absorption of such a boson by a xenon atom, these dark matter candidates would deposit an energy equivalent to their rest mass in the detector. This is the first direct detection experiment exploring the vector super-WIMPs in the mass range between 40 and 120 keV. With the use of 165.9 day of data, no significant excess above background was observed in the fiducial mass of 41 kg. The present limit for the vector super-WIMPs excludes the possibility that such particles constitute all of dark matter. The absence of a signal also provides the most stringent direct constraint on the coupling constant of pseudoscalar super-WIMPs to electrons. The unprecedented sensitivity was achieved exploiting the low background at a level 10(-4) kg-1 keVee-1 day-1 in the detector.

Dithymine photodimers and photodecomposition products of thymidylyl-thymidine induced by ultraviolet radiation from 150 to 300 nm.

Solid thymidylyl-(3'-->5')-thymidine (dTpdT) was irradiated in a vacuum with monochromatic photons from 150 to 300 nm; the photoproducts were analyzed quantitatively by high-performance liquid chromatography. The results of the experiment were as follows: (1) above 210 nm the major photoproducts were three dithymine photodimers [the cis-syn and trans-syn cyclobutane thymine dimers and the thymine (6-4) photoproduct]; below 210 nm, they were three photodecomposition products (thymine, thymidine 5'-monophosphate and thymidine 3'-monophosphate). This shows that 210 nm is the wavelength at which the major photoproducts change from dithymine photodimers (far-UV type) to photodecomposition products (X-ray type). (2) The yields of the three dithymine photodimers had a similar wavelength dependence with each other: the yields had a peak at 260 nm and gradually decreased toward shorter wavelengths to 150 nm. (3) The yields of the three photodecomposition products also had a very similar wavelength dependence with each other: the yields increased exponentially with a decrease in the wavelength. (4) The average ratios of the yield of the (6-4) photoproduct to that of the cis-syn dimer were 0.30 between 170 and 220 nm, but 0.16 between 240 and 290 nm.

Wavelength dependence of inactivation and membrane damage to Saccharomyces cerevisiae cells by monochromatic synchrotron vacuum-uv radiation (145-190 nm).

Using an electron storage ring as a source of radiation, the wavelength dependence of inactivation and membrane damage in yeast cells (Saccharomyces cerevisiae) was investigated in the range from 145 to 254 nm, with special reference to the effects of vacuum-uv radiation. The cells were irradiated on a Millipore filter in a moist chamber filled with water vapor (deoxygenated) at saturation pressure. Fluence-survival curves taken at 5-nm intervals were generally sigmoidal. Action spectra of the two types of effects were nearly identical in shape. The maximum occurred in both spectra at 160 nm, decreasing sharply toward 180 nm. The spectra remarkably resembled the calculated absorption spectrum of (liquid) water in the range from 145 to 170 nm; the spectra had no similarity at all to the absorption spectra of DNA, proteins, or lipids. These data support the theory that inactivation of wet cells by vacuum-uv radiation may be attributable to damage in the cell membrane initiated by the absorption of water molecules. Above 210 nm the spectrum for inactivation paralleled the absorption of DNA. Genetic changes (induction of gene conversion) were also observed above 210 nm. Photoreversion for the induced convertants was detectable only above 220 nm. These characteristics are consistent with the expectation that above 210 nm the site of major lethal damage shifts to DNA.

Comparisons of the effects of vacuum-uv and far-uv synchrotron radiation on dry yeast cells of different uv sensitivities.

Far-uv-sensitive (rad l/rad l) and wild-type cells of diploid Saccharomyces cerevisiae were irradiated in vacuum at 155, 170, 220, and 250 nm using synchrotron radiation (SR). Inactivation, gene conversion at leu l, and membrane damage as judged by methylene blue penetration were measured. Radiations of all these wavelengths killed dry yeast cells. In the vacuum uv, radiation at 155 and 170 nm induced membrane damage but not gene conversion, whereas far-uv radiation at 220 and 250 nm induced gene conversion but not membrane damage. The far-uv-sensitive strain showed no enhanced sensitivity to vacuum-uv radiation. These results indicate that damage to the cell membrane is considerably more important than to nuclear DNA for yeast cell inactivation by vacuum-uv radiation.

Inactivation action spectra of Bacillus subtilis spores with monochromatic soft X rays (0.1-0.6 nm) of synchrotron radiation.

Five types of Bacillus subtilis spores differing in DNA repair and recombinational capacities were exposed in vacuum to monochromatic soft X rays from synchrotron radiation. The inactivation rate constants were obtained from exposure-survival curves upon irradiations at 12 wavelengths in the range of 0.1000 nm (12.40 keV) to 0.6000 nm (2.066 keV). Spores of two repair-deficient strains, UVS (uvrA ssp) and UVP (uvrA ssp polA), exhibited almost equal sensitivities to those of wild-type UVR+, while those of two recombination-deficient strains, RCE (recE) and RCF (recF), exhibited higher sensitivities in the whole wavelength range. This suggested that the repair of DNA damage produced by soft X rays was dependent on the recombinational capabilities. Inactivation action spectra based on photon fluence showed that the effectiveness of the radiation increased as the wavelengths became longer. Abrupt changes in the effectiveness occurred around the wavelengths corresponding to the absorption edges of K-shell electrons of phosphorus and calcium. In both cases, the sensitivity was the highest at the wavelengths of the resonance absorption peak, the next highest at those of the higher energy, and the lowest at the lower energy. Mass energy absorption coefficients of spores were obtained from the transmission of a flake made of spores. They were used to derive inactivation action spectra based on absorbed doses. In these spectra, basal levels of the sensitivity seemed constant, and enhancements of the sensitivity were observed consistent with the absorption by calcium and phosphorus. Thus calcium and phosphorus atoms were the predominant targets for the absorption events leading to the inactivation of spores in the wavelength range examined.

Synchrotron system for monochromatic uv irradiation (greater than 140 nm) of biological material.

An irradiation system of monochromatic uv radiation down to the wavelength of 140 nm was constructed for biological irradiation experiments in the vacuum-uv range using synchrotron radiation (SR) from the electron storage ring. The system consists of premirror chamber, vacuum-uv monochromator, irradiation chamber, and vacuum systems. Along with the detailed description of all components of the system, the installation at the storage ring and the performance characteristics are presented.

An approach to the use of synchrotron radiation for investigating radiation effects on cultured mammalian cells in the range from 160 to 254 nm.

An experimental system with a humidified irradiation chamber was developed to investigate the effects of monochromatic synchrotron radiation on cultured mammalian cells (HeLa). The dependence of the sensitivity on the wavelength based on D0 showed a minimum at about 190 nm in the range of 160 to 254 nm.

Action spectra for inactivation of dry phage T1 after monochromatic (150-254 nm) synchrotron irradiation in the presence and absence of photoreactivation and dark repair.

Dry phage T1 was irradiated with monochromatic uv radiation (150-254 nm) in vacuo. The inactivation sensitivity was highest at 150 nm. On Escherichia coli Bs-1, the phage inactivation sensitivity was two and four times higher at 150 nm than at 254 and 230 nm, respectively. Action spectra for phage inactivation on both E. coli B and Bs-1 fit the absorption spectrum of phage DNA, except around 190 nm. The host-cell- reactivable fraction for vacuum-uv radiation (below 190 nm) was smaller than that with far-uv radiation. There was almost no photoreactivation at 150 nm, in contrast to a photoreactivation sector of about 0.3 at 254 nm.

Wavelength dependence (150-290 nm) of the formation of the cyclobutane dimer and the (6-4) photoproduct of thymine.

The action cross sections for the formation of the cyclobutane dimer and the (6-4) photoproduct of thymine as well as the absorption cross sections of thymine were determined in the wavelength region between 150 and 290 nm. Thymine films sublimed on glass plates were irradiated by monochromatic photons in a vacuum; the induced photoproducts were quantitatively analyzed by high-performance liquid chromatography (HPLC). Under our conditions, two major peaks appeared on the HPLC chromatograms of irradiated samples. The two peaks were identified as being the cis-syn cyclobutane dimer and the (6-4) photoproduct, based on their HPLC retention times, absorption spectra in the effluent, and photochemical reactivity. The fractions of the two photoproducts increased linearly with the fluence at low fluences over the entire wavelength range. Their action cross sections were determined by the slopes of the linear fluence response curve at 10 nm intervals between 150 and 290 nm. The two action spectra showed a similar wavelength dependence and had a maximum at 270 nm as well as two minor peaks at 180 and 220 nm, at which wavelengths the peaks of the absorption spectrum of thymine sublimed on a CaF2 crystal plate appeared. The quantum yields had relatively constant values of around 0.008 for the dimer and 0.013 for the (6-4) photoproduct above 200 nm, decreasing to 0.003 and 0.006, respectively, at 150 nm as the wavelength became shorter.

Inactivation action spectra of Bacillus subtilis spores in extended ultraviolet wavelengths (50-300 nm) obtained with synchrotron radiation.

Five types of Bacillus subtilis spores (UVR, UVS, UVP, RCE, and RCF) differing in repair and/or recombinational capabilities were exposed to monochromatic radiations at 13 wavelengths from 50 to 300 nm in vacuum. An improved biological irradiation system connected to a synchrotron radiation source was used to produce monochromatic UV radiation in this extended wavelength range with sufficient fluence to inactivate bacterial spores. From the survival curves obtained, the action spectra for the inactivation of the spores were depicted. Recombination-deficient RCE (recE) and RCF (recF) spores were more sensitive than the wild-type UVR spores in the entire range of wavelengths. This was considered to mean that DNA was the major target for the inactivation of the spores. Vacuum-UV radiations of 125-175 nm were effective in killing the spores, and distinct peaks of the sensitivity were seen with all types of the spores. Insensitivities at 190 and 100 nm were common to all five types of spores, indicating that these wavelengths were particularly impenetrant and absorbed by the outer layer materials. The vacuum-UV peaks centering at 150 nm were prominent in the spores defective in recombinational repair, while the far-UV peaks at around 235 and 270 nm were prominent in the UVS (uvrA ssp) and UVP (uvrA ssp polA) spores deficient in removal mechanisms of spore photoproducts. Thus, the profiles of the action spectra were explained by three factors; the penetration depth of each radiation in a spore, the efficiency of producing DNA damage that could cause inactivation, and the repair capacity of each type of spore.

Wavelength dependent formation of thymine dimers and (6-4) photoproducts in DNA by monochromatic ultraviolet light ranging from 150 to 365 nm.

We investigated the wavelength dependence of cyclobutane thymine dimer and (6-4)photoproduct induction by monochromatic UV in the region extending from 150 to 365 nm, using an enzyme-linked immunosorbent assay with two monoclonal antibodies. Calf thymus DNA solution was irradiated with 254-365 nm monochromatic UV from a spectrograph, or with 220-300 nm monochromatic UV from synchrotron radiation. Thymine dimers and (6-4)photoproducts were fluence-dependently induced by every UV below 220 nm extending to 150 nm under dry condition. We detected the efficient formation of both types of damage in the shorter UV region, as well as at 260 nm, which had been believed to be the most efficient wavelength for the formation of UV lesions. The action spectra for the induction of thymine dimers and (6-4)photoproducts were similar from 180 to 300 nm, whereas the action spectrum values for thymine dimer induction were about 9- and 1.4-fold or more higher than the values for (6-4)photoproduct induction below 160 nm and above 313 nm, respectively.

Comparisons of spore dosimetry and spectral photometry of solar-UV radiation at four sites in Japan and Europe.

In order to develop monitoring and assessment systems of biologically effective doses of solar-UV radiation, concurrent measurements of spectral photometry and spore dosimetry were conducted in summer months at four sites in Japan and Europe. Effectiveness spectra were derived by multiplying spectral irradiance in 0.5 nm steps between 290 and 400 nm with the inactivation efficiency of the spores determined using monochromatic radiation of fine wavelength resolution. Shapes of the effectiveness spectra were very similar at the four sites exhibiting major peaks at 303.5, 305.0, 307.5 and 311.0 nm. The dose rates for spore inactivation from direct survival measurements and from calculations by the integration of the effectiveness spectra were compared for 174 data points. The ratios (observed/calculated) of the two values were concordant with a mean of 1.26 (+/- 0.24 standard deviation [SD]). The possible causes for the variations and slightly larger observed values are discussed.

Experimental correspondence between spore dosimetry and spectral photometry of solar ultraviolet radiation.

The biologically effective dose of solar UV radiation was estimated from the inactivation of UV-sensitive Bacillus subtilis spores. Two types of independent measurements were carried out concurrently at the Aerological Observatory in Tsukuba: one was the direct measurement of colony-forming survival that provided the inactivation dose per minute (ID/min) and the other was the measurement of the spectral irradiance by a Brewer spectrophotometer. To obtain the effective spectrum, the irradiance for each 1 nm wavelenght interval from 290 to 400 nm was multiplied with the efficiency for inactivation derived from the inactivation action spectrum of identically prepared spore samples. Integration of the effective spectrum provided the estimate for ID/min. The observed values of ID/min were closely concordant with the calculated values for the data obtained in four afternoons in 1993. The average ratio (+/- SD) between them was 1.24 (+/- 0.16) for 14 data points showing high inactivation rates (> 0.05 ID/min). Considering difficulties in the absolute dosimetry of UV radiation, the concordance was satisfactory and improved credibility of the two types of monitoring systems of biologically effective dose of solar UV radiation.

Somatic-cell mutation induced by UVA and monochromatic UV radiation in repair-proficient and -deficient Drosophila melanogaster.

Near-ultraviolet light (UVA: 320-400 nm) constitutes a major part of sunlight UV. It is important to know the effect of UVA on the biological activities of organisms on the earth. We have previously reported that black light induces somatic-cell mutation in Drosophila larvae. To investigate which wavelength of the UVA is responsible for the mutation we have now carried out a series of monochromatic irradiations (310, 320, 330, 340, 360, 380 and 400 nm) on Drosophila larvae, using the large spectrograph of the National Institute for Basic Biology (Okazaki National Research Institutes, Okazaki, Japan). Mutagenic activity was examined by the Drosophila wing-spot test in which we observe mutant wing hair colonies (spots) on the wings of adult flies obtained from the treated larvae. The induction of mutation was highest by irradiation at 310 nm and decreased as the wavelength became longer. Neither the 380 nor the 400 nm light was mutagenic. Excision repair is known to protect cells from UV damage. In the excision-repair-deficient Drosophila, the mutagenic response induced by 310 nm irradiation was 24-fold higher than that of the wild-type (7.2 +/- 1.5 spots/wing/kJ vs 0.3 +/- 0.08 spots/wing/kJ), and at 320 nm the difference of the response was 14-fold (0.21 vs 0.015 +/- 0.005). In the case of irradiation at 330 and 340 nm the difference of the response was only two-fold (at 330 nm, 6.9 +/- 2.9 x 10(-3) vs 3.1 +/- 1.1 x 10(-3) spots/wing/kJ; at 340 nm, 3.5 +/- 0.9 x 10(-3) vs 2.0 +/- 0.7 x 10(-3). These results suggest that the lesion caused in the larvae by 320 nm irradiation may be similar to the damage induced by 310 nm and that the lights of 330 and 340 nm may induce damage different from the lesions induced by shorter-wavelength lights.

DNA damage induced by vacuum and soft X-ray photons from synchrotron radiation.

The recent development of irradiation systems using synchrotron radiation (SR) as a source is enabling researchers to obtain intense monochromatic photons having a narrow bandwidth in the vacuum-UV (VUV) and soft X-ray regions. We can thus systematically study the photon energy dependence of DNA damage formation in these energy regions. The photon energy dependence provides useful information about how energy-absorbing modes--excitations, so-called superexcitations, outer- and inner-shell ionizations--affect the type and amount of DNA damage. Furthermore, low energy electrons produced by low energy photons through photoelectric interactions are useful for studying how the electron energy affects the induction of DNA damage. A report is given on the present status of the SR irradiation systems in Japan as well as some results concerning the formation of DNA damage, in vitro and in vivo, by monochromatic photons in the VUV and soft X-ray regions.

Effects of the K-shell X-ray absorption of phosphorus on the scission of the pentadeoxythymidylic acid.

The scission of pentadeoxythymidylic acid, d(pT)5, by monochromatic soft X-rays on (2153 eV) and below (2147 eV) the K-shell absorption peak of phosphorus was studied as a model of strand breakage in DNA. Samples dried on glass plates were irradiated by monochromatic soft X-rays in vacuo, and the products were analysed by HPLC. The main products, in ascending order of the retention time, were thymine, 5'-dTMP, d(pT)2, d(pT)3, d(pT)4, d(pTp) and three unknown products (UK 1, 2 and 3), which were presumed to be d(pTpTp), d(pTpTpTp) and d(pTpTpTpTp), respectively. No difference between 2147 and 2153 eV irradiation in the nature of the induced products was detected, indicating that the K-shell absorption of phosphorus and the following Auger process produced the same types of products as those produced by photoabsorption at the other shells of phosphorus and at other atoms. The cross-sections for the induction of products at 2153 eV were 3.3-4.0 times larger than those at 2147 eV, the ratios of these values being scattered around the ratio (3.65) of absorbance of the sample between 2153 and 2147 eV. The dependence on the X-ray energy, however, almost disappeared after conversion from exposure to absorbed dose; the ratios of the G-values (number of products per 100 eV) of the products were 0.92-1.11. Photoabsorption at the K-shell of phosphorus induced products comparable or slightly less effectively than photoabsorption at the others. These results indicate that the K-shell absorption of phosphorus and the following Auger process do not have any characteristic effect on strand breakage in dry DNA, either qualitatively or quantitatively.

On the experimental distinction between ssbs and dsbs in circular DNA.

PURPOSE: To determine directly the minimal distance between two ssbs on complementary strands in circular DNA that are not observed as a dsb by electrophoresis. MATERIALS AND METHODS: 3.2 kbp DNAs with cohesive overhangs of various lengths were systematically generated by a newly devised method and electrophoresed in agarose slabs. RESULTS: At 4 degrees C, 3.2 kbp DNA with cohesive overhangs larger than 6 bp migrated as circular DNA. The minimal overhang size for the DNA to migrate as circular DNA was larger at 25 degrees C. Whether the DNA migrated as a circular or a linear molecule depended also on the nucleotide sequence of its overhangs, most notably at the minimal size. CONCLUSIONS: The minimal distance obtained in the present study agrees with the smaller values of previous indirect estimates. The dependence of the distance on experimental conditions suggests feasibility of obtaining the stagger-size distribution of radiation-induced dsbs.

Biological effects of Auger processes of bromine on yeast cells induced by monochromatic synchrotron X-rays.

The biological effects of inner-shell ionization in bromine atoms incorporated into DNA in the form of bromodeoxyuridine monophosphate (BrdUMP), induced by monochromatized synchrotron X-rays, were studied using a deoxythymidine monophosphate (dTMP)-permeable mutant of yeast, Saccharomyces cerevisiae. The BrdUMP-incorporated yeast cells were irradiated with monochromatic X-rays of 13.51 or 13.45 keV, between which the bromine K-absorption edge (13.47 keV) is located. The cells were 1.07 times more sensitive to irradiation by 13.51 keV X-rays than at 13.45 keV, while dTMP-incorporated cells did not show any difference in sensitivity. In the presence of a radioprotector during irradiation, BrdUMP-incorporated cells showed a larger enhancement (1.20). These enhancements observed in the bromine-incorporated cells cannot be explained only by an increase of the absorbed dose due to a substitution of CH3 group of thymine by bromine. It may be concluded that a major part of the enhancement was caused by inner-shell photoionization, followed by an Auger cascade of the bromine in the DNA. The quantum yield of lethality caused by the photoabsorption of bromine K-shell is not affected by the presence of cysteamine, suggesting the biological enhancement by the Auger processes may not be influenced by chemical protection.

Uracil-DNA glycosylase produces excess lethal damage induced by an Auger cascade in BrdU-labelled bacteriophage T1.

T1 phages with and without 5-bromo-2'-deoxyuridine (BrdU) labelling were irradiated in solids with monochromatic X-rays at 12.40 and 13.51 keV, below and just above the K-absorption edge of bromine (13.47 keV) in vacuum and wet states. Irradiated phages were assayed on uracil-DNA glycosylase (Udg) deficient (ung-1) and sufficient (ung+) host strains of Escherichia coli, in order to investigate the nature of the lethal damage induced by Auger cascade following X-ray absorption at the K-shell of bromine as a key atom. The results were: (1) An Auger-specific enhancement (1.15) was observed only when BrdU-labelled phages were irradiated in the wet state and assayed on ung+ host cells. (2) A Udg-specific enhancement was observed only for BrdU-labelled phages, not for unlabelled phages. (3) The sensitivities of BrdU-unlabelled phages were almost the same, despite the irradiation states and strains of the host cell, indicating that this sensitivity was a common fraction of the sensitivity under all conditions. (4) The lethal damage for the T1 phage was categorized into four fractions according to the sensitivities under different conditions: the general fraction was defined as being the sensitivity of unlabelled phages (G-fraction); BrdU-specific, but unrecognizable by Udg (B-fraction); Udg specific, but not Auger-specific (U-fraction); and Auger-specific (A-fraction). (5) Although the so-called indirect action of water radicals increased only the G-fraction by about three-fold, the B- and U-fractions were not affected by any change in the irradiation states, thus indicating that these two fractions were caused by the so-called direct action.

Effects of K-shell X-ray absorption of intracellular phosphorus on yeast cells.

The effects of K-shell absorption of phosphorus atoms on yeast cells were investigated using synchrotron X-rays that were tuned to the resonance absorption peak (2153 eV). Three types of cellular effect (cell inactivation, induction of gene conversion at the trp-5 locus, and cell membrane impairment (changes in the permeability] were measured. It was demonstrated that the enhancement factor was 1.4 at the resonance peak regarding both lethality and the induction of gene conversion in reference of off-peak irradiation (2146 and 2160 eV). No difference was found between the two off-peak irradiation energies. No cell membrane impairment was detected, irrespective of the X-ray photon energies employed within the fluence range tested. These results strongly suggest that K-shell X-ray absorption in the resonance mode by cellular phosphorus atoms causes significantly more cellular effects than the off-resonance mode of absorption, probably via some specific changes induced in the phosphates of the DNA strand. Calculations using the number of phosphorus atoms in a defined size of the trp locus (2127 base pairs) on the DNA and the absorption cross-section of the resonance mode of phosphorus showed that gene conversion is inducible at a rate of 0.13 per X-ray photon absorption per locus. These results are discussed regarding the modes of K-shell photoabsorption.