Radioisotope dating with a cyclotron

By considering radioisotope dating as a problem in trace element detection, and by using the cyclotron as a high-energy mass spectrometer for this purpose, we have shown that one can greatly increase the maximum age that can be determined while simultaneously reducing the size of the sample required. The cyclotron can be used to detect atoms or simple molecules that are present at the 10(-16) level or greater. For (14)C dating one should be able to go back 40,000 to 100,000 years with 1- to 100-mg carbon samples; for (10)Be dating, 10 to 30 million years with 1-mm(3) to 10-cm(3) rock samples; for tritium dating, 160 years with a 1-liter water sample. The feasibility of the technique has been demonstrated experimentally by measuring the tritium/deuterium ratio in a sample 24 years old. For samples many half-lives old, the fractional error in the age is small even if rates of production or deposition of the isotopes. Although cyclotrons are expensive to build, their operating costs are relatively low. If several samples are dated per hour the cost per date may not be substantially higher than it is today for decay dating. There are already more than 50 cyclotrons in operation which have the potential to do radioisotope dating, and their application to important problems of dating and trace element analysis should prove very fruitful.     

Chromatid breakage: cytosine arabinoside-induced lesions inhibited by ultraviolet irradiation

Exposure to ultraviolet light can reduce the frequency of chromatid breaks induced by cytosine arabinoside in the DNA synthetic and post-DNA synthetic phases of the cell cycle. This effect can be correlated temporally with a decrease in the uptake of tritiated thymidine after ultraviolet treatment, implying that the genesis of such breakage is intimately related to DNA synthesis and that such synthesis is not confined to the DNA synthetic phase.     

Response to RAG-mediated VDJ cleavage by NBS1 and gamma-H2AX

Genetic disorders affecting cellular responses to DNA damage are characterized by high rates of translocations involving antigen receptor loci and increased susceptibility to lymphoid malignancies. We report that the Nijmegen breakage syndrome protein (NBS1) and histone gamma-H2AX, which associate with irradiation-induced DNA double-strand breaks (DSBs), are also found at sites of VDJ (variable, diversity, joining) recombination-induced DSBs. In developing thymocytes, NBS1 and gamma-H2AX form nuclear foci that colocalize with the T cell receptor alpha locus in response to recombination activating gene (RAG) protein-mediated VDJ cleavage. Our results suggest that surveillance of T cell receptor recombination intermediates by NBS1 and gamma-H2AX may be important for preventing oncogenic translocations.     

Chloroplast DNA from tobacco leaves

DNA from tobacco leaf chloroplasts was isolated as a single component with a buoyant density in CsCl of 1.702 compared to 1.697 for nuclear DNA. 5-Methylcytosine is present in nuclear DNA but absent in chloroplast DNA. Chloroplast DNA, with a guanine-cytosine content of 43 percent, has a melting temperature of 86 degrees C and renatures completely on slow cooling, whereas nuclear DNA ( melting temperature, 84 degrees C; guanine-cytosine content, 40 percent) does not renature. About 9 percent of the total DNA in tobacco leaves is chloroplast DNA representing about 4.7 xX 10-(15) gram of DNA per chloroplast with a molecular weight of approximately 4 xX 10(7).     

Kinetics of the fe2+-mg, order-disorder reaction in anthophyllites: quantitative cooling rates

The kinetics of the Fe(2+)-Mg, order-disorder phenomenon in a highly ordered natural anthophyllite have been determined over the temperature range from 400 degrees to 720 degrees C at a pressure of 2 kilobars. At temperatures of 600 degrees C and above, equilibrium is attained by disordering as well as ordering reactions. The intracrystalline exchange is defined by a standard Gibbs free energy of 4247 +/- 54 calories per formula unit. Rate studies at 550 degrees and 500 degrees C show that equilibrium is attained by ordering but not by disordering within the same time scale and that the exchange reaction is characterized by an activation energy of approximately 55 kilocalories per formula unit. An equilibration temperature for the natural anthophyllite of 270 degrees C is determined from the termination of the ordering process owing to excessively slow reaction kinetics after approximately 10(7) years. From the rate constants of the exchange process, for different crystallization temperatures, the apparent equilibration temperature of 270 degrees C defines a maximum linear cooling rate for the rock of 1 x 10(4) degrees C per year.     

Reaction of plutonium dioxide with water: formation and properties of PuO(2+x)

Results show that PuO(2+x), a high-composition (x 相似文献   

Homograft tolerance in mice: use of urethan and sublethal irradiation

Adult mice [CBA/J (H-2(k))], which received either a single sublethal dose of x-radiation (500 rad) or urethan plus 500 rad, were given intravenous injections of C3H/HeJ (H-2(k)) spleen or bone marrow cells (18 to 42 x 10(6) cells per mouse) or both, for 3 days. C3H/HeJ tail-skin homografts were retained (over 130 days) by these mice, whereas BALB/cJ (H-2(d)) homografts all were rejected within 33 days. Similarly irradiated or urethan-treated controls (or controls treated with a combination of both), which did not receive C3H cells, rejected both homografts. Specific homograft tolerance is induced in adult mice by this procedure.     

Thermoperiodic control of diapause in an insect: theory of internal coincidence

Females of the parasitic wasp Nasonia vitripennis raised from the egg stage in the total absence of light but subjected to daily temperature cycles (13 degrees to 23 degrees C), are able to distinguish a "short-day" thermoperiod ( 13 hours at 23 degrees C per day) from a "long-day" thermoperiod ( 13 hours at 23 degrees C per day) and produce diapausing or developing progeny accordingly.     

Interaction of chromatid breaks produced by x-rays and radiomimetic compounds

The results of combination treatments of the roots of Vicia faba with certain radiomimetic compounds (8-ethoxycaffeine, maleic hydrazide, beta-propiolactone, potassium cyanide) and x-ray as well as combination treatments of certain radiomimetic compounds with one another were observed to determine whether interaction will occur between chromosomal breaks induced by different agents. Interaction was observed between breaks induced by x-rays and all of the breaks induced by chemicals but not between breaks induced by any two chemicals. The results are discussed in terms of possible breakage bond differences and the effects of temporal and spatial differences in breaks induced by different agents.     

Atmospheric oxygen: isotopic composition and solubility fractionation

Atmospheric oxygen has been found to be enriched in oxygen-18 by 23.5 +/- 0.3 per mil relative to average ocean water (SMOW). Oxygen dissolved in seawater is further enriched in oxygen-18 by 0.85 per mil at 0 degrees C. The temperature dependence of the solubility enrichment is given by epsilon (per mil) = 0.85- 0.010 t ( degrees C). This result is in good agreement with earlier measurements of the solubility effect in distilled water.     

Weakening of dunite by serpentine dehydration

A shear press has been used to determine the mechanical behavior of serpentinized dunite and forsterite at normal pressures to 50 kilobars, temperatures to 900 degrees C; and strain rates from 10(-1) to 10(-4) per second. The shear strength of dunite, containing less than 5 percent by volume of serpentine, is reduced by at least 30 percent as the temperature is raised from 300 degrees to 520 degrees C. Abundant kink bands develop at normal pressures above 35 kilobars at 27 degrees C and at lower pressures as the temperature is increased.     

Footwall refrigeration along a detachment fault: implications for the thermal evolution of core complexes

Oxygen isotope compositions of epidote and quartz from chloritic breccias that underlie the detachment fault in the metamorphic core complex of the Whipple Mountains yielded quartz-epidote fractionations that range from 4.1 to 6.4 per mil and increase systematically toward the fault. These fractionations give mean temperatures that decrease from approximately 432 degrees C at 50 meters below the fault to approximately 350 degrees C at 12 meters below the fault. This extreme thermal gradient of 82 degrees C over 38 meters (2160 degrees C per kilometer) is best explained by advective heat extraction by means of circulating surface-derived fluids. Models of lithospheric extension consider only conductive cooling resulting from tectonic denudation and thus require revision to include fluid-induced fault-zone refrigeration.     

Evidence for nuclear emissions during acoustic cavitation

In cavitation experiments with deuterated acetone, tritium decay activity above background levels was detected. In addition, evidence for neutron emission near 2.5 million electron volts was also observed, as would be expected for deuterium-deuterium fusion. Control experiments with normal acetone did not result in tritium activity or neutron emissions. Hydrodynamic shock code simulations supported the observed data and indicated highly compressed, hot (10(6) to 10(7) kelvin) bubble implosion conditions, as required for nuclear fusion reactions.     

DNA ligase: structure, mechanism, and function

DNA ligase of E. coli is a polypeptide of molecular weight 75,000. The comparable T4-induced enzyme is somewhat smaller (63,000 to 68,000). Both enzymes catalyze the synthesis of phosphodiester bonds between adjacent 5'-phosphoryl and 3'-hydroxyl groups in nicked duplex DNA, coupled to the cleavage of the pyrophosphate bond of DPN (E. coli) or ATP (T4). Phosphodiester bond synthesis catalyzed by both enzymes occurs in a series of these discrete steps and involves the participation of two covalent intermediates (Fig. 1). A steady state kinetic analysis of the reaction-catalyzed E. coli ligase supports this mechanism, and further demonstrates that enzyme-adenylate and DNA-adenylate are kinetically significant intermediates on the direct path of phosphodiester bond synthesis. A strain of E. coli with a mutation in the structural gene for DNA ligase which results in the synthesis of an abnormally thermolabile enzyme is inviable at 42 degrees C. Although able to grow at 30 degrees C, the mutant is still defective at this temperature in its ability to repair damage to its DNA caused by ultraviolet irradiation and by alkylating agents. At 42 degrees C, all the newly replicated DNA is in the form of short 10S "Okazaki fragments," an indication that the reason for the mutant's failure to survive under these conditions is its inability to sustain the ligation step that is essential for the discontinuous synthesis of the E. coli chromosome. DNA ligase is therefore an essential enzyme required for normal DNA replication and repair in E. coli. Purified DNA ligases have proved to be useful reagents in the construction in vitro of recombinant DNA molecules.     

Quenchable high-pressure polymorph of zinc selenate

A new quenchable high-pressure form of zinc selenate (ZnSeO(4)) was produced by subjecting the low-pressure modification to 40 kilobars at 400 degrees C for 30 minutes. The new form is orthorhombic, space group D(2h),(17)-Cmcm. The cell constants at 29 degrees C are: a, 5.511 angstroms; b, 8.110 angstroms; and c, 6.585 angstroms. The calculated density is 4.70 grams per cubic centimeter in comparison with 4.61 grams per cubic centimeter for the low-pressure modification. This implies a volume change of 2 percent at the transition.     

Solubility in water of normal c9 and c10, alkane hydrocarbons

A new method for equilibrating water containing alkane hydrocarbons with a gas phase and analyzing the gas for hydrocarbon content by gas chromatography extends analytical sensitivity to better than 0.1 part per billion. The solubilities at 25 degrees C of the normal C(9) (220 parts per billion) and C(10) (52 parts per billion) alkanes decrease with increasing carbon number. A discontinuity occurs at the normal C(11), alkane, probably because of a change from true solubility (molecular dispersion) to accommodation (aggregation).     

Survival of mouse embryos frozen to -196 degrees and -269 degrees C

Mouse embryos survived freezing to -196 degrees C. Survival required slow cooling (0.3 degrees to 2 degrees C per minute) and slow warming (4 degrees to 25 degrees C per minute). Depending on the specific rates used, 50 to 70 percent of more than 2500 frozen and thawed early embryos developed into blastocysts in culture after storage at -196 degrees C for up to 8 days. When approximately 1000 of the survivors, including some frozen to -269 degrees C (4 degrees K), were transferred into foster mothers, 65 percent of the recipients became pregnant. More than 40 percent of the embryos in these pregnant mice gave rise to normal, living full-term fetuses or newborn mice.     

Water and carbon in rusty lunar rock 66095

Lunar rock 66095 contains a hydrated iron oxide and has an unusual amount of water for a lunar rock (140 to 750 parts per million), 90 percent of which is released below 690 degrees C. The deltaof water released at these low temperatures varies from -75 to -140 per mil relative to standard mean ocean water (SMOW). The small amount of water released between 690 degrees and 1300 degrees C has a delta of about -175 +/-25 per mil SMOW. These delta values are not unusual for terrestrial water. The delta(18)O of water extracted from 110 degrees to 400 degrees C has a value of +5+/- I per mil SMOW, similar to the value for lunar silicates from rock 66095 and different from the value of -4 to -22 per mil found for samples of terrestrial rust including samples of rusted meteoritic iron. The amount of carbon varies from 11 to 59 parts per million with a delta(13)C from -20 to -30 per mil relative to Pee Dee belemnite. Only very small amounts of reduced species (such as hydrogen, carbon monoxide, and methane) were found, in contrast to the analyses of other lunar rocks. Although it is possible that most of the water in the iron oxide (goethite) may be terrestrial in origin or may have exchanged with terrestrial water during sample return and handling, evidence presented herein suggests that this did not happen and that some lunar water may have a deltaD that is indistinguishable from that of terrestrial water.     

Field measurement of slow metamorphic reaction rates at temperatures of 500 degrees to 600 degrees C

High-temperature metamorphic reaction rates were measured using strontium isotopic ratios of garnet and whole rock from a field site near Simplon Pass, Switzerland. For metamorphic conditions of cooling from 612 degrees +/- 17 degrees C to 505 degrees +/- 15 degrees C at pressures up to 9.1 kilobars, the inferred bulk fluid-rock exchange rate is 1.3(-0.4)(+1.1) x 10(-7) grams of solid reacted per gram of solid per year, several orders of magnitude lower than laboratory-based estimates. The inferred reaction rate suggests that mineral chemistry may lag the evolving conditions in Earth's crust during mountain building.     

Risk of thyroid neoplasms after irradiation in childhood. Studies of populations exposed to radiation in childhood show a dose response over a wide dose range

The incidence of thyroid carcinoma and of clinically palpable thyroid nodules is compared in three groups of individuals many years after exposure to ionizing radiation in childhood or infancy. The estimated mean cumulative doses to the thyroid gland ranged from 20 rad in the Ann Arbor series irradiated with x-rays as infants for thymic enlargement upward to 1225 rad of mixed radiation (including that from ingested radioiodine) in the Marshallese children. Intermediate between these extremes is the high-risk subgroup in the Rochester series of persons irradiated for thymic enlargement; they received an estimated mean thyroid dose of 335 rad. A plot of the incidence of thyroid nodularity against the total cumulative thyroid dose gives what could be a linear dose response with no threshold or, at least, a threshold below 20 rad. The risk of developing carcinoma in the three studies ranges from 0 to 5.5 cases and that for nodularity 38 to 52 cases per 106 persons exposed per rad (thyroid dose) per year. There is evidence dence from animal experimentation that the biologically effective dose in the case of the Marshallese would have been considerably lower than the cumulative physical dose; hence, the dose response might actually be curvilinear at least in the higher dose range. In view of the uncertainties regarding dose and the assumptions made in my study, the risk values may not be exact; however, these values reflect the trends regarding risks estimated from the best available data in man. Furthermore, the estimated risk values apply only if radiation exposure occurred in childhood.