Y. Mishima

Cosmic-ray intensities under sea-water at depths down to 1400 m

SummaryThere is a disagreement between the muon energy spectrum observed at seal-level and the one evaluated from the depth-intensity relation of cosmic rays below ground by using the theoretical expression for energy loss of the muon traversing the rock. As the cause of this fact, one can point out following two reasons: uncertainties in the determination of the compositions of the rocks or imperfection of the theoretical expression for energy loss. To decide this point, the cosmic-ray intensities have been measured at various depths up to 1380 m in the ocean where the composition of the water is well known and the uniformity is well certified. The observed result is in excellent agreement with the sea-level energy spectrum in the corresponding energy region, the current energy loss formula being taken into account. From this fact, it is pointed out that the previous observations of range spectra underground suffered from ambiguities due to uncertainties in the composition of the rocks.RiassuntoC’è disaccordo fra lo spettro d’energia dei muoni osservato al livello del mare e quello valutato dalla relazione profondità-intensità dei raggi cosmici sotto terra usando l’espressione teorica per la perdita d’energia dei muoni nell’attraversare la roccia. Come causa di questo fatto si possono indicare due ragioni: incertezza nella determinazione delle composizioni delle rocce o imperfezioni nell’espressione teorica della perdita di energia. Per decidere su questo punto si sono misurate le intensità dei raggi cosmici a varie profondità sino a 1380 metri nell’oceano dove la composizione dell’acqua è ben nota e l’uniformità è ben certa. Il risultato ottenuto è in eccellente accordo con lo spettro d’energia al livello del mare nel corrispondente intervallo di energia, tenendo conto della corrente formula per la perdita d’energia. In base a questo fatto si rileva che le precedenti osservazioni degli spettri d’energia sotto terra subiscono le ambiguità dovute alle incertezze nella composizione delle rocce.

Rapid recovery of anomalous cosmic ray flux at 1 Au in solar cycle 22

A new observation of low energy cosmic ray particles with the energies from 4 to 120 MeV/n by the HEP instrument onboard the GEOTAIL satellite orbiting at 1 AU shows a remarkable enhancement of anomalous cosmic ray (ACR) N, O and Ne in the period September 1992–December 1993 before approaching to the solar minimum in solar cycle 22. The ACR fluxes obtained by the GEOTAIL observation are in good agreement with the results earlier obtained from the SAMPEX satellite [Mewaldt et al. 1993a,b]. The ACR carbon is also evident though the C enhancement is small compared with those of N, O and Ne. We confirm the new SAMPEX finding that the ACR fluxes have recovered more rapidly in 1992–1993 than in previous solar cycles and find this tendency continues to the end of 1993.

Composition and energy spectra of anomalous cosmic rays observed by the GEOTAIL satellite

Abstract The composition and energy spectra of anomalous cosmic-rays (ACRs) in the energies 6 – 200 MeV/n have been measured during the period from September 1992 to August 1995 using the High Energy Particle instrument onboard the GEOTAIL satellite orbiting at 1 AU. A remarkable enhancement of ACR N, O, Ne and Ar is observed during the period. A flux enhancement of anomalous argon at 1 AU is confirmed by the GEOTAIL satellite. The first possible evidence for the emergence of a measurable sulfur component in ACR below about 20 MeV/n is found. The flux increase of anomalous sulfur with 10.4 eV FIP is smaller than that of ACR carbon with 11.3 eV FIP, and much smaller than those of high FIP elements, which suggests that the fraction of neutral carbon and sulfur atoms is significantly lower in the very local interstellar medium.

Energy loss straggling of heavy charged particles in thick silicon absorbers

The energy loss straggling of heavy charged particles with relatively high energies passing through thick uniform Si detectors (1.0 mm–9.2 mm) has been studied in a wide range of the ratio Δ E / E 0 where Δ E is the energy loss and E 0 is the initial energy of the incident particles. The experimental results are compared with those predicted by straggling theories. It suggests that the measured distributions are in good agreement with those predicted from the Bohrs or Livingston-Bethes theories when Δ E / E 0 0.3.