• D Lal

Articles written in Proceedings – Section A

• Production and interaction of mesons at very high energies

A shower containing ∼360 charged and neutral shower particles produced by a magnesium nucleus of 2×1014e.v. provides a source of mesons whose interactions have been studied over an aggregate track length of about 3 meters in emulsion. It is first shown that in very large showers not only the incident energy can be determined but also the energy of the shower particles. The angular distribution of the shower particles agrees closely and their multiplicity is consistent with Fermi’s theory.

It is shown that the interaction cross-section of charged shower particles is geometric and that at an energy ∈ ∼ 1000µc2 they produce on the average 5 to 6 mesons. The mesons are preferentially emitted in the backward direction in the reference system in which the momentum of the incident meson balances that of a nucleon in the target. The excitation produced in the target nucleus is much lower than in nucleon-produced showers of comparable multiplicity.

The charged mesons arenot emitted in pairs by a heavier meson of short lifetime.

Charged shower particles as well as electrons produce electron-positron pairs with a cross-section of the order of that predicted by theory.

The number of neutral mesons decaying intoγ-rays in a time ≤10−14 sec. is nearly equal to the number of charged shower particles.

There exists evidence for the emission of neutral particles, different from neutrons, capable of producing meson showers.

The energy balance of the event suggests that a few shower particles carry an appreciable fraction of the total energy.

• Flux determination on high energy protons by photographic emulsion method

• The preparation of large nuclear emulsion detectors and their application to the study of K-mesons and hyperons

A method is described for constructing out of individual sheets of nuclear emulsion a detector of large volume and stopping power which can be used for observing entire chains of successive nuclear interaction and decay processes initiated by high energy particles.

Such a detector which had been exposed for several hours in the stratosphere has proved particularly valuable for detecting K-mesons and Hypersons* and for studying their modes of production and decay.

• Observations onτ-mesons and onK-mesons giving rise to capture stars

In an emulsion block detector1 exposed in the stratosphere three cases of the production ofτ-mesons in nuclear disintegrations have been observed and 5 cases of negativeK-mesons which when brought to rest are captured by nuclei. In one case the production of aτ-meson is associated with the production of a charged hyperon in the same nuclear event. In another case the production of a negativeK-meson is associated with the production of a slow positiveK-meson. The production ratio of slowτ-mesons to slow positiveπ-mesonsτ/π+=1·1+0·8% and the ratio of slow negativeK-mesons to slow negativeπ-mesonsK/π=0·95±0·5%. The mass of theτ-meson is found to be$$M_\tau = 975 \cdot 9 \pm 2 \cdot 2 m_e$$ and that ofK-mesons$$M_{K^ - } = 927 \pm 75 m_e$$ Proof is given that all three of the decay products of theτ-mesons areπ-mesons. It is suggested that allτ-mesons observed to decay in emulsions are positively charged and that theK-mesons observed by us may be the negative counterpart of this particle. Possible relations between the various phenomenologically distinguished types of chargedK-mesons are discussed.

• Production frequency of neutral pi-mesons in high energy interactions

• The preparation of large nuclear emulsion detectors and their application to the study of K-mesons and hyperons

• Errata

• On the construction of large nuclear emulsion block detectors

• Nuclear interactions of K-mesons

Fast K-mesons (τ- as well as Kw-mesons) with energies between 150 and 250 MeV can cause nuclear disintegrations and lose a substantial fraction of their kinetic energy without losing their identity. The character of the interaction in three of the four cases discussed here exhibit a remarkable degree of similarity.

The nuclear capture of a K-meson at rest is discussed. The nature and distribution of prongs in the capture star suggest that a Λ°-hyperon may have been formed during the capture process.

• The measurement of tritium activity in natural waters - Part I. Techniques

To exploit natural and artificial activity of tritium, H3 (half-life, 12.3 y.), as a tracer for the study of the large-scale circulation of air in the tropo-sphere, water balance and characteristics of ground water, systematic measurements of its activity in wet precipitations, surface and subsurface waters have been carried out since 1961. The levels of tritium activities in such samples vary over a wide range, factors of 104 or more. Due to large dilution with hydrogen of the tropospheric water, the specific activities in surface and subsurface waters are very low, and employment of sensitive detection techniques becomes necessary.

In this paper, we report on the techniques adopted for the measurement of tritium which permit an accurate determination of T/H ratios exceeding 10−17, without carrying out an electrolytic enrichment. A factor of twenty enrichment then suffices to measure natural levels of tritium in ground waters of up to 25 years “age” corresponding to T/H ratios of ca. 5 × 10−19.

• The measurement of tritium activity in natural waters - Part II. Characteristics of global fallout of H3 and Sr90

The concentrations of tritium have been determined in wet precipitations occurring over the Indian subcontinent during 1961–64, using a sensitive method for counting of tritium activity discussed in Part I* of this paper.

The tritium concentrations varied significantly during the period of observation; highest concentrations were observed during 1963. An analysis of the data reported here, in conjunction with those available for concentrations of H3 and Sr90 in rains at higher latitudes, reveals that these nuclides which were originally placed at high altitudes in the polar regions during late 1962, were deposited chiefly at 30°–90° latitudes during 1963 and 1964 respectively in relative proportions of 1 and 0·6. The data show that the largest gradients in their zonal deposition occur at about 35°–40° N latitude and that to a first approximation, their deposition per unit area in 1963 or 1964 was practically uniform, separately in the 30°–90° and 0°–30° latitude regions. This observation suggests the existence of two well-defined cells, which are internally well mixed: the meridional transport to low latitudes occurs as a result of interaction between these cells. The annual deposition rates of Sr90 as observed during 1963 and 1964 suggest a mean time of 3 months for exchange of air between the two cells, in good agreement with the values deduced for mid-months of the year on the basis of analysis of bomb produced C14 data.

The tritium and strontium data for the inland, coastal and island stations are analysed to evaluate the importance of (i) the re-evaporation of tritium from continents, and (ii) the molecular exchange of atmospheric tritium with oceanic water. Process (i) probably plays a significant role over the continents throughout the year; its effect, however, is experimentally visible only during June to September. The estimated concentration of H3 in evaporated water suggests that the precipitated water mixes very slowly with that in the soil; limits on the equivalent amount of exchangeable soil water are given.

It is shown that the relative wet deposition of H3 and Sr90 atisland andcoastal stations is similar to their estimated concentration ratio in upper level tropospheric air. Furthermore, the relative concentrations of H3 and Sr90 at continental and occanic stations differ only to the extent expected due to reinjection of H3 over continents. Thus, if one takes into account the recycling of H3 at continental stations (which results in about a 50% higher apparent deposition on an annual basis), one is led to the conclusion that process (ii) is rather unimportant; an upper limit of 30% on the fraction of tritium removed over oceans by molecular exchange is deduced.

The mean annual concentration of Sr90 in wet precipitation is lower at oceanic stations compared to that at continental stations. This could be due to meteorological effects peculiar to oceanic areas,e.g., higher rainfall and quick recycling of evaporated water. Otherwise, one must postulate a significant removal of Sr90 (and H3) by ocean spray and jet action.

• U234/U238 activity ratios in South Pacific Ocean waters

Analyses of uranium extracted from South Pacific waters by thein-situ extraction method point to a uniform depth independent value of 1·14 ±0 02 for the U234/U238 activity ratio. This value is also the same, within one standard deviation, as that determined for the N. Pacific, S. Indian and Atlantic waters.

• A study of the vestigial records of cosmic rays in lunar rocks using a thick section technique

An experimental technique has been developed for systematic measurements of fossil tracks along selected planes cut from grains and rocks. With controlled etching, the technique allows successive revelation of tracks in different minerals in the same section, a typical sequence being olivine, anorthite, clinopyroxene. It thus becomes possible to study precisely the cosmic ray track density variations over dimensions much greater than those of individual crystals. The technique also provides accurate information on the relative recording characteristics of different minerals present in a rock and cosmic ray tracks can be studied with a minimum interference of tracks due to spontaneous fission of uranium and transuranic elements.

Continuous chains of sections, each section measuring approximately 1 cm., have been cut along several different planes in fifteen rocks from Mare Tranquillitatis, Oceanus Procellarum and Fra Mauro region. The cosmic ray track measurements from these sections have provided dramatic evidence for a number of processes affecting lunar rocks. The statistical, and non-uniform nature of erosion by micrometeorite bombardment can be seen in sections intersecting exposed surface which show regions of very steep track density gradients interspersed with eroded regions having lower track densities. The thick section technique permits determination of the energy spectrum of VH nuclei from track density gradients that extend over distances limited only by the dimensions of the rock, and, more important, in samples of identical orientation. The latter is particularly important in higher energy regions (deeper within the rock) where variations in crystal orientation cause track density differences of the same order as real changes in the gradient. Also in the near surface regions of rocks where low energy particles produce steep track density gradient, the thick section method has proved indispensable since it permits accurate depth determinations not possible in the spot sampling procedure.

In this paper the technique of studying track profiles in thick sections is described. Although developed primarily for studying lunar samples, the thick section technique is also useful for similar studies in meteorites, particularly for gas-rich meteorites containing irradiated grains. In contrast to single grain studies, thick sections preserve the grain boundaries and permit accurate depth—density measurements. In addition thick section studies have revealed occasional large uniformly irradiated lithic fragments which would not have been possible to discover by spot sampling methods.

• Errata

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