SummaryThe « knock-on electron » technique has been employed to determine the energy spectrum of nuclei with chargeZ⩾6 in the primary cosmic radiation. In this technique the energy per nucleon of a heavy primary cosmic ray nucleus is determined by measuring the angles of emission and the energies of electrons which the nucleus « knocks on » in elastic collisions along its path in traversing nuclear emulsion. Since the energies of the knock-on electrons are relatively small (<100 MeV in this experiment), they can be measured with precision by the method of multiple Coulomb scattering. The conditions for reliable estimation of primary energy have been determined and the reliability of the method verified by comparison with results obtained by other methods. The charges and energies of the particles have been determined in the following two ways: (i) for particles which are arrested in the emulsion, from measurements of range andδ-ray density; (ii) for particles which do not stop, from measurements ofδ-ray density and of the energies and the angles of emission of the fast knock-on electrons; in the few cases where the particle slowed down in traversing the stack, the variation ofδ-ray density along the track was determined. Measurements were made on 291 tracks obtained in a stack flown from Iowa, (λ=53° N), at 113 000ft. for four hours on March 13, 1956. 206 of these tracks were due to particles with chargeZ⩾6. The exponent of the integral energy spectrum of the medium (6⩽Z⩽9) group of nuclei has been obtained as 1.65±0.27 in the energy range 0.23 to 9 GeV/nucleon and that of the heavy (Z⩾10) group of nuclei as 1.82±0.59 in the energy interval 0.41 to 9 GeV/nucleon. The exponent for the S-group of nuclei (Z⩾6) is then 1.78±0.24. The geomagnetic cut-off energy atλ=54.5° N has been estimated to be 230 MeV/nucleon. The values of the flux of M and H-nuclei were found to be 10.7±1.0 and 5.3±0.7 particles/m2s sr, respectively (on March 13, 1956). A comparison of these values with those obtained in other experiments shows that a Forbush type of decrease had taken place in the intensity of the heavy nuclei, similar to that observed by McDonald in the α-particle flux on the same flight; neutron monitors on ground had also recorded a Forbush decrease at the same time.RiassuntoLa tecnica degli « elettroni d’urto » è stata impiegata per determinare lo spettro dell’energia dei nuclei con caricaZ⩾6 nella radiazione cosmica primaria. In questa tecnica l’energia per nucleone di un nucleo pesante di raggi cosmici primari si determina misurando gli angoli di emissione e le energie degli elettroni che il nucleo « colpisce » nelle collisioni elastiche lungo il suo cammino attraverso l’emulsione nucleare. Poichè le energie degli elettroni d’urto sono relativamente piccole (<100 MeV in questo esperimento), possono essere misurate con precisione col metodo dello scattering multiplo di Coulomb. Si sono determinate le condizioni per una attendibile valutazione dell’energia primaria e si è verificata l’attendibilità del metodo per confronto con i risultati ottenuti con altri metodi. Le cariche e le energie delle particelle sono state determinate in due modi: 1) per le particelle che si arrestano nell’emulsione con misure del percorso e della densità dei raggiδ; 2) per le particelle che non vi si fermano, misurando la densità dei raggiδ e le energie e gli angoli di emissione degli elettroni d’urto veloci; nei pochi casi in cui le particelle rallentavano nell’attraversare lo stack, si è determinata la variazione della densità dei raggiδ lungo la traiettoria. Si fecero misurazioni su 291 tracce ottenute in uno stack di emulsioni lanciato dallo Iowa, (λ=53° N) a 113 000 ft. per quattro ore il 13 Marzo 1956. 206 di queste tracce erano dovute a particelle con caricaZ⩾6. L’esponente dello spettro integrale dell’energia del gruppo medio (6⩽Z⩽9) di nuclei si è determinato in 1.65±0.27 nel campo di energie fra 0.23 e 9 GeV/nucleone e quello del gruppo pesante (Z⩾10) di nuclei in 1.82±0.59 nell’intervallo di energie fra 0.41 e 9 GeV/nucleone. L’esponente del gruppo S (Z⩾6) di nuclei è quindi 1.78±0.24. L’energia di cut-off geomagnetico aλ=54.5° N si è stimata in 230 MeV/nucleone. Si è trovato che i valori del flusso di nuclei M ed H sono (10.7±1.0) e (5.3±0.7) particelle/m2 s sr, rispettivamente (13 Marzo 1956). Un confronto di questi valori con quelli ottenuti in altre prove mostra che ha avuto luogo un decremento di tipo Forbush nell’intensità dei nuclei pesanti, analogo a quello osservato daMcDonald nel flusso di particelleα nello stesso volo; nella stessa epoca, rivelatori di neutroni a terra hanno pure registrato un decremento di Forbush.

The Energy Spectrum of Nuclei with Charge Z 13 in the Primary Cosmic Radiation

Total and partial charge-changing cross-sections of 600 A MeV Si14+ ions in a thick polyethylene target were measured by CR39 track etch detectors using an optical microscope DM 6000 M system installed with Leica QWin Plus software. The CR39 nuclear track detectors were used to identify the incident charged particles and their fragments using the cone-height distribution method. The charge resolution of CR39 detectors was achieved to be 0.17e in the present condition. The value of the total charge-changing cross-section was calculated to be σ
 tot
 = (766±17) mb. To calculate the partial charge-changing cross-sections for ΔZ = −11, −10, ..., −1, , number of events corresponding to each fragment were determined from multiple Gaussian fitting of cone-height distributions within 95.5% confidence level and the number of incident and survived beam ions were counted within 99.7% confidence level. Possible odd-even pattern in the values of the partial charge-changing cross-section is also presented. The measured charge pick-up cross-section for ΔZ = +1 is (7.2 ±1.7) mb.

Fragmentation cross-section of 600 A MeV Si14+ ions in thick polyethylene target

Calibration of CR39 detectors with new system for Fe26+ ion beam and measurement of total charge changing cross-section in Al target

Fragmentation cross sections of 788A MeV 28Si on carbon and polyethylene targets

A stack consisting of CR-39 (HCB 0.5%), nuclear emulsions and X-ray films was exposed to primary cosmic rays by a balloon lauched from Alice Springs in 1983 and was flown for 32 hours at an atmospheric depth 9.8 g cm−2 air. The recovered plastic plates were etched in 7.5 N NaOH solution at 80°C for 96 hours. The major and minor axes of the elliptic etch pits were analysed. About 1112 elliptic etch pits were scanned. The measured integral flux of very heavy (VH) and Fe nuclei above 3.5 GeV/n are in approximate aggreement with the earlier survey of Dokeet al., but yield a flatter energy spectrum when compared to the recent high-energy extrapolated spectrum of Zatsepinet al. The charges of the detected heavy nuclei were also confirmed from delta-ray counting of heavily ionized tracks in nuclear emulsions. The energy of the incident heavy nuclei has been measured from the distribution of opening angles of alpha fragments initiated by heavy nuclei in nuclear emulsions.

Estimation of chemical abundance of primary cosmic-ray nuclei over Alice Springs using balloon-borne passive detectors

The production of fragmented nuclei from relativistic56Fe beam available from LBL Bevalac at 1.88A GeV has been studied using CR-39 (DOP) passive detector placed at an angle of 60° with respect to the beam. The histogram showing the experimental frequency distribution of minor axes of the elliptic etch pit shows the presence of the fragmented nuclei produced with charge numberZ from 25 up to 21. The histogram further reveals the presence of nuclei withZ=27 and 28. The production of nuclei heavier than56Fe is possibly due to the charge exchange or pick-up phenomena.

Fragmentation of56Fe in Al at 1.88A GeV

A stack of CR-39 (HCB 0.5%) passive solid-state nuclear track detectors were irradiated by primary cosmic rays near the top of the atmosphere. The stack of detectors were flown by balloons launched from Alice Springs, Australia in 1983. After 16 h 41 min flight exposure at an atmospheric pressure of 9.5 mb the plates were collected and chemically etched in 6.7 N NaOH aqueous solution at 70°C for 27 hours. The diameter distribution of the 1208 etch pits caused by the presence of heavy ions in primary cosmic rays was measured by a transmitted light Leitz Ortholux microscope having an objective×10 and a×15 filar micrometer eyepiece attachment. The diameter distribution was standardised and calibrated against the conventional cosmic-ray results found by Tasakaet al. The estimated charge spectrum is in accord with the nuclear emulsion data of Daniel and Durgaprasad, Kristianssonet al., Saito and Chohanet al.

Chemical composition of the primary cosmic rays near the top of the atmosphere

The Hungarian brand CR-39(MA-ND) plastic has been irradiated with a 84Kr ion beam of energy 0.45A GeV and etched for four different etching times, viz. 4, 6, 8 and 12 h. The estimated charge resolution of a CR-39(MA-ND) detector for registering the nuclei 32 ⩽ Z⩽ 36 was found to be 0.18e which is close to our previous observation of the response with a CR-39(DOP) Pershore made plate exposed to a 1.88A GeV 56 Fe beam at the Lawrence Berkeley Laboratory's Bevalac. It was found that the estimated etch rate ratio V T V G is independent of etching time. The cone length and minor axis of the etch pits has been found to increase with etching time.

A. K. Ganguly

Papers

Fragmentation of56Fe in Al at 1.88A GeV

Chemical composition of the primary cosmic rays near the top of the atmosphere

Charge resolution of a Hungarian brand CR-39(MA-ND) detector exposed to A 84Kr beam of energy 0.45A GeV