Stable Isotopes Of Magnesium Research Articles

Study of the possibility of using data on the content of stable isotopes of magnesium to predict the region of apple production

Relevance. Reliable criteria for determining the region of origin of fruit products attract the constant attention of researchers in various countries. A number of approaches aimed at solving this problem are already known, and some progress has been made in interpreting experimental data for fruits on the content of stable isotopes of hydrogen, carbon, oxygen, nitrogen, and sulfur. It is possible that information about the fractionation of stable isotopes of magnesium will allow us to obtain equally important and reliable information to achieve the same goals.Methods. A mass spectrometer with inductively coupled plasma, standard samples of stable isotopes of magnesium produced in Russia were used to perform the work. The total concentration of magnesium cations was determined by atomic absorption in the flame and on the capillary electrophoresis system.Results. During the research, the mass and percentage ratio of three stable isotopes (24Mg, 25Mg, 26Mg) in the studied samples of apples grown in different soil and climatic zones was established. The data obtained reflect the relative prevalence of isotopes of this element, expressed in the ratio of the least common isotope to the most common. The deviation of the isotopic composition of Mg prototypes from the conventional standard was determined, significant deviations were obtained for the magnetic isotope 25Mg and non-magnetic 26Mg. The initial data on the fractionation of stable isotopes of magnesium in raw materials growing in different regions were obtained, with the aim of possible subsequent use as indicators in biochemical and physiological studies. A combination of the obtained information on Mg isotopes is assumed, taking into account the content of isotopes of other metals, which will make it possible to specify the region of growth of raw materials with greater reliability.

Isotopic Composition and Fractionation of Stable Magnesium Isotopes in Relation to Fruits Growing in Different Regions of Russia

Background: Present study on determining the region of origin of raw materials, the authenticity of organic food products is relevant. The determination of the natural content of stable isotopes are not able to answer these questions. The solution can be obtained by studying the ratios and fractionation of stable isotopes, which provide information on the transformations of chemical elements in biological, ecological and geochemical studies. Methods: During 2020-2021 obtained data on the content of magnesium isotopes in apple fruits of 14 varieties from three growing regions of the Russian Federation. Previously, over 10 years, a database was formed of more than 500 samples on the level of potassium, calcium, magnesium, phosphorus and nitrogen cations, depending on the variety and phase of fruit development. Result: Research presents the results of determining the natural content of stable magnesium isotopes in apples, the isotopic composition and their fixation, depending on different regions of the growth of raw materials. The isotopic composition of apples (26Mg/24Mg) varies from 0.1669047 to 0.1782649; the deviation of the magnesium isotopic composition from the conventional standard varies within 25.52-43.52; their chemical fractionation has value: in Central Zone of Russia-185.2-212.3; South of Russia-248.0-311.5; North Caucasus-182.7-214.8.

Filling the gap in Ca input-output budgets in base-poor forest ecosystems: The contribution of non-crystalline phases evidenced by stable isotopic dilution

In terrestrial ecosystems, plant-available pools of magnesium and calcium are assumed to be stored in the soil as exchangeable cations adsorbed on the surface of mineral and/or organic particles. The pools of exchangeable magnesium and calcium are measured by ion-exchange soil extractions. These pools are sustained in the long term by the weathering of primary minerals in the soil and atmospheric inputs. This conceptual model is the base of input-output budgets from which soil acidification and the sustainability of soil chemical fertility is inferred. However, this model has been questioned by data from long-term forest ecosystem monitoring sites, particularly for calcium. Quantifying the contribution of atmospheric inputs, ion exchange and weathering of both primary, secondary and non-crystalline phases to tree nutrition in the short term is challenging.In this study, we developed and applied a novel isotopic dilution technique using the stable isotopes of magnesium and calcium to study the contribution of the different soil phases to soil solution chemistry in a very acidic soil. The labile pools of Mg and Ca in the soil (pools in equilibrium with the soil solution) were isotopically labeled by spraying a solution enriched in 26Mg and 44Ca on the soil. Labeled soil columns were then percolated with a dilute acid solution during a 3-month period and the isotopic dilution of the tracers was monitored in the leaching solution, in the exchangeable (2 sequential 1molL−1 ammonium acetate extractions) and non-crystalline (2 sequential soil digestions: oxalic acid followed by nitric acid) phases.Significant amounts of Mg and Ca isotope tracer were recovered in the non-crystalline soil phases. These phases represented from 5% to 25% and from 24% to 50%, respectively, of the Mg and Ca labile pools during the experiment. Our results show that non-crystalline phases act as both a source and a sink of calcium and magnesium in the soil, and contribute directly to soil solution chemistry on very short-term time scales. These phases are very abundant in acid soils and, in the present study, represent a substantial calcium pool (equivalent in size to the Ca exchangeable pool). The gradual isotopic dilution of Mg and Ca isotope ratios in the leaching solution during the experiment evidenced an input flux of Mg and Ca originating from a pool other than the labile pool. While the Mg input flux originated primarily from the weathering of primary minerals and secondarily from the non-crystalline phases, the Ca input flux originated primarily from the non-crystalline phases. Our results also show that the net calcium release flux from these phases may represent a significant source of calcium in forest ecosystems and actively contribute to compensating the depletion of Ca exchangeable pools in the soil. Non-crystalline phases therefore should be taken into account when computing input-output nutrient budgets and soil acid neutralizing capacity.

Fractionation and assimilation of Mg isotopes by fungi is species dependent.

Symbiotic ectomycorrhizal fungi mobilize nutrients from both organic and inorganic substrates and supply them to their host plants. Their role in mobilizing base cations and phosphorus from mineral substrates through weathering has received increasing attention in recent years but the processes involved remain to be elucidated. We grew selected ectomycorrhizal and nonmycorrhizal fungi in axenic systems containing mineral and organic substrates and examined their capacity to fractionate and assimilate stable isotopes of magnesium. The mycorrhizal fungi were significantly depleted in heavy isotopes with the lowest Δ26 Mg values (the difference between δ26 Mg in fungal tissue and δ26 Mg in the substrate) compared with nonmycorrhizal fungi, when grown on mineral substrates containing granite particles. The ectomycorrhizal fungi accumulated significantly higher concentrations of Mg, K and P than the nonmycorrhizal fungi. There was a highly significant statistical relationship between δ26 Mg tissue signature and mycelial concentration of Mg, with a clear separation between most ectomycorrhizal fungi and the nonmycorrhizal fungi. These results are consistent with the idea that ectomycorrhizal fungi have evolved efficient mechanisms to mobilize, transport and store Mg within their mycelia.

Nuclear spin catalysis in living nature

Experiments with cells enriched in stable magnesium isotopes, magnetic 25Mg or nonmagnetic 24Mg and 26Mg, are carried out. It is revealed that adaptation of bacteria E. coli to the growth media enriched in magnetic 25Mg proceeds faster as compared to the growth media enriched in nonmagnetic magnesium isotopes. In experiments with another commonly accepted cell model, S. cerevisiae yeast, it is revealed that the rate constant of postradiation recovery of the cells after UV irradiation is two times higher for cells enriched in 25Mg than for cells enriched in the nonmagnetic isotope. In collaboration with Ukrainian colleagues from the Palladin Institute of Biochemistry, the effects of different isotopes of magnesium on ATPase activity of myosin isolated from myometrium are studied. It is found that the rate of the enzymatic hydrolysis of ATP for 25Mg is 2.0–2.5 times higher as compared to nonmagnetic isotopes 24Mg and 26Mg. Some possible mechanisms of magnetic isotope effects (nuclear spin catalysis) in biological objects are discussed.

Isotope Concentrations from 24-h Urine and 3-h Serum Samples Can Be Used to Measure Intestinal Magnesium Absorption in Postmenopausal Women1–3

Studies suggest a link between magnesium status and osteoporosis. One barrier to more conclusive research on the potential relation is measuring intestinal magnesium absorption (MgA), which requires the use of stable isotopes and a ≥6-d stool or 3-d urine collection. We evaluated alternative methods of measuring MgA. We administered 2 stable magnesium isotopes to 15 postmenopausal women (cohort 1) aged 62 ± 8 y with a dietary magnesium intake of 345 ± 72 mg/d. Participants fasted from 1200 h to 0700 h and then consumed breakfast with ∼23 mg of oral26Mg and ∼11 mg of i.v.25Mg. We measured magnesium isotope concentrations in 72-h urine, spot urine (36, 48, 60, and 72 h), and spot serum (1, 3, and 5 h) samples collected after isotope dosing. We calculated MgA using the dose-corrected fraction of isotope concentrations from the 72-h urine collection. We validated new methods in 10 postmenopausal women (cohort 2) aged 59 ± 5 y with a dietary magnesium intake of 325 ± 122 mg/d. In cohort 1, MgA based on the 72-h urine collection was 0.28 ± 0.08. The 72-h MgA correlated most highly with 0–24 h urine MgA value alone (ρ = 0.95,P < 0.001) or the mean of the 0–24 h urine and the 3-h (ρ = 0.93,P < 0.001) or 5-h (ρ = 0.96,P < 0.001) serum MgA values. In cohort 2, Bland-Altman bias was lowest (−0.003,P = 0.82) using means of the 0–24 h urine and 3-h serum MgA values. We conclude that means of 0–24 h urine and 3-h serum MgA provide a reasonable estimate of 72-h MgA. However, if researchers seek to identify small changes in MgA, we recommend a 3-d urine or extended stool collection. This trial was registered atclinicaltrials.gov> as NCT01593501.

The dynamics of calcium and magnesium inputs by throughfall in a forest ecosystem on base poor soil are very slow and conservative: evidence from an isotopic tracing experiment (26Mg and 44Ca)

Using nutrient budgets, it has been proven that atmospheric deposition of Mg and Ca sustains the fertility of forest ecosystems on base-poor soils. However the fate of this nutrient input within the ecosystem was presently unknown. Our hypothesis is that the biological cycling of these nutrients is very rapid and conservative to prevent further Mg and Ca losses most especially in ecosystems on base-poor soils. Stable isotopes of magnesium and calcium (26Mg and 44Ca) were used to trace the dynamics of throughfall Mg and Ca in the forest soil of a 35-year-old beech stand. The aim of the present study was to (1) understand the processes and the velocity of the incorporation of tracers in the biogeochemical cycles and (2) compute Mg and Ca budgets for the ecosystem by isotope dilution. Rainfall Mg and Ca were strongly and rapidly retained mainly by ion exchange in the thin OL litter-layer. However, Ca was much more strongly retained in the litter-layer than Mg. As a result, 2 years after the application of tracers (2012), 92 % of 26Mg and 67 % of 44Ca was released and transferred to the soil or taken up by trees. The vertical transfer of Mg was very slow only 15 % of 26Mg was found below 15 cm depth in 2012. Ca was slower than 26Mg only 9 % of 44Ca was found below 5 cm depth. Although matrix flow was the main vertical transfer process of Ca and Mg, preferential transfer in macropores occurred. Overall, Mg was more rapidly leached through the soil profile than Ca because the soil CEC was mainly composed of organic charges which affinity for Ca is much higher than for Mg. 27 % of 26Mg and 20 % of 44Ca was found in tree biomass and total tracer recovery was close to 100 %. These results suggest that no tracers were lost to drainage over the 2 years. Finally, applying the isotopic dilution theory to the whole-ecosystem enabled us to estimate Mg and Ca budgets −0.9 kg ha−1 year−1 for Mg, which was close to computed input–output budgets −0.8 and 0 kg ha−1 year−1 for Ca, which was very different from input–output budgets (−3.1 kg ha−1 year−1). Our results suggest that a Ca source is underestimated or not taken into account. Over all, organic matter of the litter-layer and in the soil profile played an essential role in the retention of throughfall Mg and Ca and their cycling within the forest ecosystem.

Resonance neutron-capture cross sections of stable magnesium isotopes and their astrophysical implications

We have measured the neutron capture cross sections of the stable magnesium isotopes 24,25,26Mg in the energy range of interest to the s process using the neutron time-of-flight facility n_TOF at CERN. Capture events from a natural metal sample and from samples enriched in 25Mg and 26Mg were recorded using the total energy method based on C62H6 detectors. Neutron resonance parameters were extracted by a simultaneous resonance shape analysis of the present capture data and existing transmission data on a natural isotopic sample. Maxwellian-averaged capture cross sections for the three isotopes were calculated up to thermal energies of 100 keV and their impact on s-process analyses was investigated. At 30 keV the new values of the stellar cross section for 24Mg, 25Mg, and 26Mg are 3.8±0.2 mb, 4.1±0.6 mb, and 0.14±0.01 mb, respectively.5 MoreReceived 9 January 2012DOI:https://doi.org/10.1103/PhysRevC.85.044615©2012 American Physical Society

Calibration of the ISOLDE acceleration voltage using a high-precision voltage divider and applying collinear fast beam laser spectroscopy

A high-voltage divider with accuracy at the ppm level and collinear laser spectroscopy were used to calibrate the high-voltage installation at the radioactive ion beam facility ISOLDE at CERN. The accurate knowledge of this voltage is particularly important for collinear laser spectroscopy measurements. Beam velocity measurements using frequency-comb based collinear laser spectroscopy agree with the new calibration. Applying this, one obtains consistent results for isotope shifts of stable magnesium isotopes measured using collinear spectroscopy and laser spectroscopy on laser-cooled ions in a trap. The long-term stability and the transient behavior during recovery from a voltage dropout were investigated for the different power supplies currently applied at ISOLDE.

Measurement of photoionization cross section from the 3s3p 1P1 excited state of magnesium

The photoionization cross section from the 3s3p 1P1 excited state has been measured in the energy region from the first ionization threshold up to 1.4 eV excess energy using a two-step photoionization and saturated ionization technique in conjunction with an atomic beam source and a time-of-flight mass spectrometer that enables the separation of the three stable isotopes of magnesium on the time axis. The absolute value of the photoionization cross sections from the 3s3p 1P1 excited state near the 3s ionization threshold has been measured as 90 ± 16 Mb (at 354.5 nm ionizing wavelength) for the dominating isotope (24Mg) whereas the value at the peak of the 3p2 1S0 auto-ionizing resonance has been determined as 785 ± 141 Mb. The present experimentally measured photoionization cross sections are compared with the existing experimental and theoretical work, showing excellent agreement.

Meal effect on magnesium bioavailability from mineral water in healthy women

Magnesium intakes in many industrialized countries are below recommended daily allowances. Magnesium-rich mineral water may contribute to coverage of magnesium requirements by providing significant amounts of natural, energy-free, bioavailable magnesium. The objectives were to determine magnesium bioavailability from magnesium-rich (110 mg/L) mineral water in healthy subjects when consumed alone and to evaluate the effect of simultaneous meal consumption. Magnesium bioavailability was measured in 10 healthy women with the use of a crossover design. Stable magnesium isotopes ((25)Mg and (26)Mg) were administered orally with mineral water, which was consumed with or without a meal. Apparent magnesium absorption was determined by fecal monitoring, and magnesium retention was determined from urinary excretion of magnesium isotopes. The mean (+/-SD) magnesium absorption from mineral water consumed alone was 45.7 +/- 4.6% (range: 40.2-55.5%) and was significantly greater (P = 0.0001) when it was consumed with a meal (52.3 +/- 3.9%; 46.2-60.2%), a relative difference of 14.4%. Magnesium retention also was significantly greater (P = 0.0004) when mineral water was consumed with a meal (41.5 +/- 4.2%; 35.2-50.6%) than when consumed alone (37.4 +/- 4.0%; 33.1-47.0%), a relative difference of 11.0%. In healthy young women, approximately 50% of the magnesium from magnesium-rich mineral water was absorbed when consumed alone. Magnesium bioavailability from mineral water is enhanced when the water is consumed with a meal, perhaps because of a slower gastrointestinal transit time, the presence of digestion products from the meal, or both. Regular consumption of magnesium-rich mineral water could make a valuable contribution to magnesium requirements.

Determination of isotope enrichments of magnesium in microwave-digested biological samples by thermal ionization mass spectrometry using a direct loading technique

The isotope ratios of magnesium were determined in isotopically normal and 26 Mg-enriched samples of human blood, blood plasma, urine and faeces and bovine muscle. The measurements were made with a magnetic sector, thermal ionization mass spectrometer (TIMS) equipped with a multiple ion collector system for simultaneous detection of the ion currents. The samples were decomposed using microwave digestion with HNO3 and HCI. Without further chemical treatment, the mineralized samples were deposited together with silica gel and boric acid on rhenium filaments, which served as thermal ionization source filaments. This method, called the direct loading technique (DLT), results in stable ion signals of the magnesium isotopes with isotope ratios indistinguishable from those of natural Mg standards within experimental error. Fractionation-corrected 26 Mg/24 Mg ratios of natural Mg standards were determined with a relative external precision of 0.02%. The magnesium recoveries for all of the analysed matrices were > or = 97%; 26 Mg was added to calibrated sample solutions to produce isotopic enrichments within a range typically appearing in samples of human tracer studies. Linear regression analysis of measured versus expected per 1000 (/1000) enrichments yields y = 0.998x + 0.79. The DLT described here is a simpler and quicker method than other methods reported hitherto. It has the advantage of avoiding magnesium separation and purification steps prior to TIMS analysis for all of the analysed biological samples and thus reduces contamination and guarantees optimum magnesium recovery. The reported method improves the applicability of stable isotopes of magnesium in human tracer studies.

Accurate measurement of stable isotopes of magnesium in biological materials with inductively coupled plasma mass spectrometry.

A method is described for the accurate isotopic determination of magnesium (24Mg, 25Mg, 26Mg) in biological materials, which is based on inductively coupled plasma mass spectrometry (ICP-MS). The analytical performance of the method was examined with respect to the requirements of stable isotope tracer studies. When applied to the measurement of base-line isotope ratios (MRa/b) in rat tissue the following results were obtained (data are given in terms of MR25/24 and MR26/24± 1RSD, n= 4 or 5): standard solution of Mg (0.05 µg ml–1), 0.1394 ± 0.6, 0.1697 ± 0.8; bone, 0.1401 ± 0.5, 0.1706 ± 0.8; brain, 0.1404 ± 0.2, 0.1720 ± 0.3; kidney, 0.1392 ± 0.5, 0.1702 ± 0.6; liver, 0.1388 ± 0.4, 0.1696 ± 0.5; muscle, 0.1396 ± 0.2, 0.1716 ± 0.4; plasma, 0.1385 ± 0.3, 0.1691 ± 0.3; red cells, 0.1383 ± 0.1, 0.1694 ± 0.2; and urine, 0.1402 ± 0.1, 0.1721 ± 0.3. The measurement precision for replicate analyses of each matrix was in the range 0.1–1.0% and the mean value of the isotope ratio for different matrices agreed with the corresponding ratio for the standard solution to within 1.5%. When measured over a 10-h period, the isotope ratios appeared to be independent of the observed drifts in the ion beam intensities. The instrument blank contribution to the ion beam intensities was about 0.1% of the values obtained for a solution containing 0.05 µg ml–1 of natural Mg. The absolute detection limit for Mg (based on the experimental standard deviation of blanks run over 10 h) was <2 ng for all three stable isotopes. The ICP-MS phase of the analysis was capable of making 50–150 separate measurements of both ratios in a 10-h period, depending on the desired level of measurement precision within the range 0.1–1.0%. A complete analytical scheme for the accurate determination of the three stable isotopes of Mg, which is based on precipitation with ammonium phosphate, is described. The accuracy of the method was tested using the standard reference materials Bovine Liver (NBS 1577a, 600 ± 15 µg g–1 of Mg) and Animal Bone (IAEA H-5, 3550 ± 90 µg g–1). The proposed method provided the following data (µg g–1): SRM 1577a, 617 ± 4 and IAEA H-5, 3585 ± 16. For other biological matrices of interest, the accuracy of the method was compared with atomic absorption spectrometry. The complete analytical procedure, up to the point of mass spectrometric measurement, can be performed on about 20 samples per working day.

Elemental isotopic abundance determinations of magnesium in biological materials by secondary ion mass spectrometry.

Secondary ion mass spectrometry is applied to the determination of stable magnesium isotopes. The method involves the high mass solution determinations of the isotopes from air-dried films of untreated and acid-treated biological fluids. Calibration curves are used for the isotopic quantifications. Agreements of less than 2.5% enrichments of /sup 26/Mg were determined for spiked and unspiked acid-treated samples. Differences of about 5%, 2%, and 1% enrichments can be determined with 95% certainty in analyses times of 2, 15, and 40 min, respectively. 23 references, 3 figures, 6 tables.

The isotopic composition of solar flare accelerated magnesium

We report observations of energetic solar particles from the 1978 September 23 solar flare in which the three stable isotopes of magnesium are clearly resolved with a mass resolution of 0.23 amu. We find ^(25)Mg/^(24)Mg = 0.14 (+0.05, -0.02) and ^(26)Mg/^(24)Mg = 0.15 (+0.04, -0.03) in the energy interval 12-36 MeV per nucleon. These observations are consistent with terrestrial magnesium isotopic abundances.

Isobaric analog impurities from neutron capture and transmission by magnesium

Resonance neutron interactions with the stable isotopes of magnesium were measured at the Oak Ridge electron linear accelerator time-of-flight facility. The transmission of a natural metal sample (78.7% $^{24}\mathrm{Mg}$) was measured at 200 m and capture by natural metal and by enriched isotope samples was measured at 40 m. Twenty-six resonances in $^{24}\mathrm{Mg}+n$ up to 1.8 MeV were fitted with Breit-Wigner multilevel parameters. The data were sufficient to assign spin and parity to 19 of these. The capture data were analyzed for resonances up to 850 keV for $^{24}\mathrm{Mg}+n$, 265 keV for $^{25}\mathrm{Mg}+n$ (${17}^{+}$ resonances), and 440 keV for $^{26}\mathrm{Mg}+n$ (4 resonances). Average capture at stellar interior temperatures was calculated. The $^{24}\mathrm{Mg}+n$ data serve to assess the isospin impurities in three isobaric analog states. Three other states exhibit reduced neutron widths each several percent of the Wigner limit which may be understood in terms of simple shell model configurations.NUCLEAR REACTIONS $\mathrm{Mg}(n,x)^{24,25,26}\mathrm{Mg}(n,\ensuremath{\gamma})$, $E=0.1\ensuremath{-}1800$ keV; measured $T(E)$ and ${\ensuremath{\sigma}}_{n,\ensuremath{\gamma}}(E)$; deduced $^{25,26,27}\mathrm{Mg}$ level parameters, isobaric analog isospin impurities, shell model configurations, average capture in stars as a function of temperature.

Elastic electron scattering from the stable isotopes of magnesium

Elastic electron scattering cross sections from the isotopes 24Mg, 25Mg and 26Mg have been measured in the momentum transfer range 0.20-1.15 fm-1. The individual RMS radii and also the differences between the isotopes are investigated. The measured form factors are compared with the predictions of an extended Nilsson model calculation.

Cross Sections of Stable Magnesium Isotopes Produced in Aluminum by Proton

Cross sections for production of stable magnesium isotopes by proton induced reactions in aluminum have been measured by mass spectrometry. Magnesium containing a few nano grams(10-9g)of induced products was separated from the target, by solvent extraction of aluminum followed by cation exchange purification for magnesium. Average cross sections for a reaction 27Al(p, α)24Mg are 140±30mb and 140±30mb in the ranges of 15-11 MeV and 11-6 MeV protons respectively, and for a reaction 27Al(p, 2p)26Mg, 50±10mb by 15-11 MeV p.

Nuclear Reactions in Stars. IV. Buildup from Carbon

Reactions leading to the formation of C, O, and Ne at temperatures T of about 10/sup 8/ deg K, in the core of red giant stars, were studied previously. The nuclear reactions and resulting nucleogenesis are now studied for a gas consisting of C/sup 12/, O/sup 16/, and Ne/sup 20/ at 6 x 10/sup 8/ to 10 x 10/s up 8/ deg K and densities around 10/sup 4/ g/cc. The basic reaction rates are calculated and a set of simultaneous differential equations for various abundances as a function of time is solved numerically. The C/sup 12/ is destroyed by compound nucleus formation from C + C collisions in about 10/sup 5/ and 1 years, respectively, at 6 x 10/sup 8/ and 8.5 x 10/sup 8/ deg K. The net result is the production of some additional amounts of O/sup 16/ and Ne/sup 20/; appreciable amounts of Na/sup 23/ and the three stable magnesium isotopes (mainly Mg/sup 24/) and decreasing amounts of Al/sup 27/, Si, etc. The ratio of Na/sup 23/ to Mg/sup 24/ produced is almost 1/2, appreciably larger than the cosmic ratio. During the carbon burning, protons and alphas are released. At temperatures below 7 x 10/sup 8/ degmore » K, neutrons are produced very copiously through the sequence C/sup 12/(p, gamma )N/sup 13/; N/sup 13/ yields C/sup 13/; C/sup 13/( alpha ,n)O/sup 16/. If small amounts of metals i n the Fe-region (up to about one metal nucleus per 1000 C/sup 12/ nuclei) were present originally, each metal nucleus will absorb about 30 neutrons, At temperatures above 8 x 10/sup 8/ deg K, N/sup 13/ is photodisintegrated and the neutron production is appreciably less. (auth)« less