Edgardo Picciotto

Mesure des isotopes du thorium dans l'eau de mer

In order to study the geochemistry of thorium isotopes in the hydrosphere, particularly in the ocean, a method has been worked out by which Th-232 (thorium)λ, Th-230 (ionium), Th-228 (radiothorium), and Th-227 (radioactinium) can be determined separately. Eight samples of 20 to 40 litres of sea-water, from 23·0% to 34·97%, salinity, were collected in November 1953, in the Skagerak and the Gullmarfjord (Sweden). Thorium was isolated by the following procedure: just after collection, the samples were brought to pH 2 and a given amount of Th-234 (UX1) was added as tracer. Thorium was first precipitated with Fe(OH)3 as carrier. Further purification was obtained by ion-exchange column chromatography followed by solvent extraction; the final fraction was obtained as the citric complex, a form suitable to incorporation in the photographic emulsion. The total yield varied from 8 to 23% according to the sample, as determined by the β-activity of the tracer. The various thorium isotopes were measured through their α-activity, using nuclear photographic emulsions, more precisely the double-emulsion technique. RdTh and RdAc both generate five-branched stars; more than 90% of these originated from RdTh, as indicated by the length of the tracks: while Io and Th only yield single tracks of range 18·8 μ and 15 μ respectively in the emulsion. Most samples showed a much lower activity than expected; this did not make it possible to discriminate between Io and Th through the range distribution of their tracks, thus we could only ascertain upper limits of Io and Th concentrations. Average concentrations corresponding to a total volume of 140 litres of water are as follows (in grams per ml): RdTh = (4.0 ± 1.4). 10−21 Th < 2.10−11 RdAc < 7.10−23 Io < 6.10−16 In one of these samples (salinity: 33·7%) we have found an Io concentration of 26.10−16 g/ml. This high value is attributed to a nonhomogeneous distribution of Io in the sea. Before the conclusions are drawn, we must point out the following restriction: 1. (1) Our water samples, including those in the oceanic range of salinity, were not collected in an oceanic environment, as all were taken in coastal waters. 2. (2) Our experimental results should correspond to the total thorium content of the samples. It must be pointed out, however, that a thorium fraction which both would not exchange with UX1 at pH 2 and would not coprecipitate with Fe(OH)3 would remain undetected with our procedure. We assume the following concentration for the other radioactive elements: U = 1·5. 10−9g/ml, Ra = 0·8.10−16 g/ml, Th < 6.10−12 g/ml. The state of radioactive equilibrium between two nuclides A and B shall be denned by their activity ratio: RA/B = λA. NA/λB. NB The following conclusions can be drawn from the above data: 1. (1)RIo/U-238 < 0.02. More than 98% of the Io resulting from U-238 disintegration in the oceann cannot be accounted for. This lack of Io in the sea-water must be correlated with the presence of unsupported Io in the deep-sea sediments. These two corroborating facts definitely prove the hypothesis of ionium precipitation with the sediments. 2. (2) RIo/Ra < 0.15. Ra is in excess by a factor of 6 with respect to its equilibruim with Io. This could possibly result from the redissolution of part of the Ra originating from this Io of the sediments. 3. (3)The average RdTh concentration of 4.10−21g/ml should correspond to an equilibrium concentration of 2.6.10−11 g/ml of the Th-232. We have, however, shown that, in at least two samples, RdTh is far over its equilibrium value with Th. Indeed, if we assume Th 4. We can only account for this surprising result by supposing that the excess RdTh results from an excess of its parent MsTh, (Ra-228) brought in by rivers or redissolved from the sediments. Owing to the short half-life of both these nuclides, such a RdTh excess should be found only in the vicinity of the shore or the bottom. 4. (4)RRdAc/U-235 < 0·1. More than 90% of the RdAc from U-235 in the ocean cannot be accounted for. Considering the short half-life of RdAc, this suggests that actinium or protactinium are precipitated with the sediments together with the Io. 5. (5)In both U-238 and Th-232 families, a radium isotope (Ra and MsTh) appears to be in excess over its parent thorium-isotope (Io and Th). The presence in the ocean of unsupported Ra(T = 1600 years) and MsTh (T = 6·7 years) is of great interest. A study of the distribution of these isotopes should yield valuable data on their diffusion rates and on deep currents. As far as radioactive geochemistry is concerned, the ocean is characterized by extremely low concentrations of nuclides of all three radioactive families and by the total disruption of the radioactive equilibrium in these families. A calculation of the geochemical balance of radioactive elements in the hydrosphere from the above data is given in the last part of the paper.

Lead-210 and strontium-90 in an Alpine glacier

Abstract 210 Pb and 90 Sr measurements were carried out on firn and ice samples from the Kesselwandferner (Austrian Alps). 90 Sr seems to be selectively lost in the firn with respect to the other fission products and to 210 Pb. The distribution of the 210 Pb ‘ages’ of the ice samples follows a consistent pattern and is in general agreement with surface velocity measurements.

Artificial radioactivity reference horizons in Greenland firn

Abstract : Total beta measurements have been made on melt water samples from a stratigraphically dated firn core profile from the inland Greenland ice sheet (77 deg 10 min N, 61 deg 08 min W). A marked increase in radioactivity is found in the 1953 firn layer which corresponds to the first important fallout from nuclear test bombs. The pre-1953 natural beta activity is 5 dpm/kg. The influx of artificial debris from the Ivy tests in 1953 is noted by a sharp rise in beta activity to 10 dpm/kg. Total 90Sr deposit to June 1964 is 9.3 plus or minus 1.5 millicuries/km2 (24 plus or minus 4 mC/mi2). Average 210Pb activity at time of deposit is 3.9 plus or minus 0.4 dpm/kg. (Author)

Chimie des neiges antarctiques et taux de deposition de matiere extraterrestre - deuxieme article

Abstract Sodium, magnesium, potassium, calcium, iron, manganese and nickel were measured in firn samples from Base Roi Baudouin (BRB, 70°S, 24°E), Amundsen-Scott Station (ASS, 90°S) and Plateau Station (PLA, 79°S, 40°E). The rates of snow accumulation at the three stations are respectively, in g.cm−2.yr−1 : 40, 6 and 2.8. The analyses were carried out by neutron activation, atomic absorption and isotope dilution. In most cases, Fe, Mn, Ni were measured simultaneously by two or three methods on aliquots of the same sample. The corrections for contamination never exceeded 10%. Average concentrations found at BRB (5 samples, 14 kg total) and at PLA (4 samples, 32 kg) respectively are as follows, in p.p.b.: Na 250-30; Mg 29-5; K 29-5; Ca 17-8; Mn 0.1-0.35; Fe 6-7; Ni 0.36-0.08. At AAS (2 samples, 3 kg) : Na 9; Mn 0.2; Ni 0.1. On account of the everpresent possibility of contamination, these values must be considered as upper limits. The following conclusions are drawn from the results: 1) The earlier Ni measurements at BRB and ASS are erroneously high. The accretion rate of extraterrestrial matter derived from these data is too high by an order of magnitude. 2) At PLA which is, amongst the three stations, the one furthest away from sources of terrestrial contamination, the fallout rate of inorganic matter (soluble as well as particulate) is of the order of 10−6g.cm−2.yr−1. The relative abundances of the measured elements are compatible with the assumption that this matter is a mixture of 98% of average crustal material and 2% of chondritic matter. In such a case, 80% of the Ni found would be of cosmic origin. If a cosmic origin and an abundance of 1.3% (the value in ordinary chondrites) are postulated for all the Ni found, an upper limit of 105 tons/yr is calculated for the influx rate of extraterrestrial matter over the earth. The deposition is assumed to be uniform. 3) At BRB, the fallout rate of inorganic matter is at least 10−4 g.cm−2.yr−1, indicating an overwhelming terrestrial contribution as it could be expected from the geographical location. Nevertheless, the Fe/Ni ratio is found to be much closer to the chondritic ratio at BRB than at PLA. 4) The upper limit of 105 tons/yr for the influx rate is an order of magnitude lower than other estimates based on the Ni, Co, Fe contents of Greenland ice. On the other hand, it is in agreement with the value derived from the Ir and Os content of deep-sea sediments. Both sets of results indicate that at the utmost 2% of the Ni in the ocean could be of cosmic origin.

Confirmation de la période du Thorium-232

RésuméLa période du232Th est déduite de l’activité du228Th mesurée par les étoiles de désintégrations successives dans les emulsions nucléaires. On trouveT= 1.39.1010 ans avec une dispersion relative de ±2 %, en parfait accord avec la valeur de Kovarik et Adams.RiassuntoDall’attività del228Th misurata per mezzo delle stelle di disintegrazioni successive nellc emulsioni nucleari si deduce il periodo del232Th Si trovaT= 1.39-1010 anni con una dispersione relativa di ±270. in porfetto aeeordo col valore di Kovarik e Adams.

Plecchroic haloes and the artificial coloration of biotites by α particles

RésuméNous avons donné dans ce travail les doses d’α nécessairesà la formation des halos pléochroiques dans des biotites de différentsgranites. Nous avons étudié seulement les halos résultant de l’action des a de la famille de l’Uranium et du Thorium en équilibre, émis en couche épaisse à partir d’inclusions grandes par rapport au parcours des a. Le début d’apparition du halo a lieu pour 1014 α émis par cm2 de l’inclusion, la saturation pour environ 5–1016 α/cm2, le début d’inversion pour 5–1017 a/cm2. Les biotites des mêmes granites ont été irradiées, expérimentalement par une source de Rn+RaA+RaC’, les a étant complètement absorbés dans la biotite. Les diverses biotites présentent des. sensibilités très différentes aux irradiations des α, l’accroissement de densité pour une même dose pouvant varier d’un facteur 5. L’évolution de la coloration suit approximativement une loi de la forme Δdmax=Δd(1−exp[−ϱα]), ϱ, Δdmax étant deux paramètres spécifiques et a la dose d’α par cm2. Le début de la coloration apparaît pour une dose de 2–1013 a/cm2 en accord, avec les anciens résultats de Joly et Rutherford sur des biotites de pegmatites. La saturation nécessite des doses de 7 à 10 · 1014 α5cm2 et l’inversion de la coloration n’apparaît pas encore pour des doses de 1.5 · 1015 a/cm2. Ces doses semblent plus faibles que celles qui sont nécessaires pour produire les mêmes effets dans les halos. Il n’est cependant pas possible de comparer directement ces doses à celles données pour les halos, à cause des différences dans le spectre d’énergie des a et dans les conditions géométriques d’irradiation. Si au lieu de comparer les doses en nombre d’α, on les compare ennombre d’ions formés, l’accroissement de la coloration reste plus rapide dans l’irradiation expérimentale que dans les halos. Les accroissements maxima de densité optique correspondant à la saturation de la coloration sont à peu près les mêmes dans les halos et dans les biotites irradiées expérimentalement. La comparaison des halos et des Motites irradiées montre qu’il existe une correspondance au moins qualitative entre les sensibilités d’une biotite aux irradiations expérimentales et naturelles. Au point de vue des mesures d’âge, il n’est pas encore possible d’utiliser ces expériences de coloration artificielle pour une détermination d’âge absolue par les halos. Pour les déterminations relatives, il faut tenir compte des différences de sensibilité. Une méthode proposée consiste à comparer entre elles des biotites présentant la même sensibilité à l’irradiation expérimentale en supposant que cette similitude de sensibilité est valable aussi pour la formation des halos.RiassuntoAbbiamo dato in questo lavoro le dosi di a necessarie alla formazione degli aloni pleiocroici nelle biotiti di differenti graniti. Abbiamo studiato soltanto gli aloni risultanti dall’azione delle α della famiglia dell’Uranio e del Torio in equilibrio, emesse in strato spesso da inclusioni grandi rispetto al percorso delle α. L’inizio della comparsa dell’alone si ha per 1014 α emesse per cm2 dell’inolusione, la saturazione per circa 5–1015 α/cm2, l’inizio dell’inversione per 5–1017 α/cm2. Le biotiti degli stessi graniti sono state irradiate sperimentalmente con una sorgente di Rn+RaA + RaC’, con completo assorbimento delle a nella biotite. Le diverse biotiti presentano sensibilità assai differenti alle irradiazioni delle a, potendo l’accrescimento di densità per una stessa zona variare di un fattore 5. L’evoluzione del colore segue approssimativamente una legge della forma Δdmax = Δd(l-exp [-ϱα]), ϱ e Δdmax sono due parametri specifici e α la dose d’α per cm2. L’inizio della colorazione appare per una dose di 2 · 1013 a/cm2 in accordo coi veechi risultati di Joly e Rutherford su biotiti pegmatitiche. La saturazione richiede dosi da 7 a 10-1014 α/cm2 e l’inversione della colorazione non si manifesta ancora per dosi di 1.5 · 1015 α/cm2. Queste dosi appaiono più deboli di quelle necessarie a produrre gli stessi effetti negli aloni. Non è tuttavia possibile confrontare direttamente quèste dosi con quelle date per gli aloni, date le differenze nello spettro di energia delle α e nelle condizioni geometriche d’irradiazione. Se anziche confrontare le dosi in numéro d’à le si confrontano innuméro di ioni formati l’accrescimento della colorazione risulta più rapido nell’irradiazione sperimentale che negli aloni. I massimi accrescimenti di densità ottica che corrispondono alla saturazione della colorazione sono circa uguali negli aloni e nelle biotiti irradiate sperimentalmente. Il confronto degli aloni con le biotiti irradiate mostra che esiste una corrispondenza almeno qualitativa tra le sensibilità d’una biotite alle irradiazioni sperimentali e alle naturali. Per la datazione non è ancora possibile utilizzare queste esperienze di colorazione artiflciale per una determinazione assoluta per mezzo degli aloni. Per le determinazioni relative bisogna tener conto delle differenze di sensibilità. Si propone un metodo consistente nel confrontare fra di loro delle biotiti dotate della stessa sensibilità all’irradiazione sperimentale, supponendo che questa similitudine di sensibilità sussista anche per la formazione degli aloni.

Étude photographique de la distribution de la radioactivité dans la granodiorite de l'Adamello

Abstract The granodiorite from Adamello (Italy) shows a distribution of the α-radioactivity similar to that found in granitic rocks. This distribution is characterized by its heterogeneity. More than 95% of the activity is concentrated in accessories which amount to 0.1% of the volume of the rock. The most active accessories are 100,000 times more active than the essential minerals. In some cases, more than 90% of the α-radioactivity, apparently connected to the essential minerals is in fact due to microscopic inclusions of accessories. The apparent enrichment in uranium and thorium of the ferro-magnesian minerals, pointed out by several authors, is due to the abundance of radioactive inclusions in those minerals and not to the substitution by uranium and thorium of certain elements of their crystalline lattice. Very radioactive inclusions have been found. Their uranium and thorium content exceeds 10%. Their exact identification is not possible, but there are good reasons to believe that the most active are uraninite and torbernite. Allanite, the most active among the usual accessories, emits 0·47Tα/cm 2 /sec. corresponding to an uranium content of 0·16%. Following order of decreasing activity, then come: titanite, zircon, epidote, apatite. The “real activity” of the essential minerals, quartz, feldspar, biotite, is of the order of 3. 10 −3 α/cm 2 /sec which would correspond to an uranium concentration of the order of 10 −7 gr/gr. In contradiction to what has been observed in other granitic rocks, Adamellos granodiorite does not show any concentration of radioactivity into boundaries between crystals. Relationships between the intensity of the pleoehroic haloes and the activity of the inclusions-which produce them are discussed.

The summer 1957-1958 at the South Pole an example of an unusual meteorological event recorded by the oxygen isotope ratios in the firn

Abstract Climatological records from the Amundsen-Scott Station, South Pole, show the summer 1957–1958 to be characterized by periods of exceptionally warm air temperature and above normal snow accumulation. The firn layer deposited during this summer displays an exceptionally high oxygen-18 content, as revealed by 18 O/ 16 O ratio profiles obtained by Epstein et al. [6,7] and by us in five pits dug along-side accumulation stakes.

Etude préliminaire des Halos Pléochroïques de quelques Roches métamorphiques et éruptives du St-Gothard

Die Untersuchung der pleochroitischen Höfe der Biotite in Orthogneisen, Paragneisen und Bündnerschiefern des südlichen Gotthardmassives ergab durchwegs ein ähnliches Alter, nämlich < 60 Millionen Jahre. Es bedeutet dies, dass eventuell vorhandene vortertiäre Höfe in prätriadischen Biotiten der Gneise während der alpinen Metamorphose fast oder ganz durch Rekristallisation zerstört worden sind.

Arrêt de la diffusion des radio-éléments dans les émulsions nucléaires par exposition a basse température

ResuméOn montre que la diffusion des radioéléments dans les émulsions nucléaires peut être arrêtée par l’action de la basse température. On n’observe plus d’effet de diffusion du Radon à −85 °C et du Tn à −36 °C. A ces températures respectives, la répartition observée entre étoiles et traces simples correspond à la répartition théorique. Cette méthode permet d’utiliser le phénomène caractéristique de la formation des étoiles pour mesure de très faibles quantités de radioéléments.RiassuntoSi mostra come la diffusione dei radioelementi nelle emulsioni nucleari può essere arrestata dall’azione della bassa temperatura. Non si osserva più alcun effetto di diffusione del Radon a −85 °C e del Tn a −36 °C. A tali temperature la ripartizione osservata tra stelle e tracce semplici corrisponde alla ripartizione teorica. Questo metodo permette d’utilizzare il fenomeno caratteristico di formazione delle stelle per la misura di debolissime quantità di radioelementi.