A Measurement of Heavy Cosmic Rays at Sea Level Using A Mass Spectorometer

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A cosmic-ray mass spectrometer has been used to detect slow (beta=v/c=0.5-0.8), heavy charged particles ; protons deuterons, tritons and long-lived particles at sea level. The mass-spectrometer (acceptance 290cm^2・sr) consists of three layers of plastic scintillation counters (each 200cmx20cmx2cm), single layer of iron absorber and four layers of two-dimensional drift chambers (1.2mx2.4m area and cell size of 5cmx5cm). The first and second scintillators (TOF counters) separated 144cm, measured velocity (beta) and charge of incident particles. The third scintillator below the absorber measured the velocity after the absorber. We have calibrated the spectorometer by cosmic-ray muons and obtained 7% beta resolution at beta=1. To reject muons, a delayed coincidence between the first and second scintillators was used. For the first observation, the absorber was 9.3cm iron, the trigger-delay was 1 or 2nsec and the observation period was about six months. After selecting good events, the char … More ge was determined by pulse heights (geometric mean on both ends) of scintillators above absorber, and the beta by the mean-time delay normalized by the track length between the scintillators. A distribution of pulse height of the scintillator below the absorber vs incident beta was obtained. Strips of slant muons, protons and deuterons were separeted clearly and on the predictions for 9.3cm iron. The strip of tritons was not seen clearly, but there were thirteen events on the prediction. The calculated fluxes were normalized by the muon intensity at sea level. The proton intensity (cm^<-2>sec^<-1>sr^<-1>) was (7.99<plus-minus>0.17) x10^<-6> (beta=0.63-0.71), a little lower than the Sydney and Auckland's results. The deuteron intensity was (1.37<plus-minus>0.15) x10^<-7> (beta=0.59-0.63), agrees with those results. The triton was (6.0<plus-minus>1.7) x10^<-8>, which is close to the Sydney's result. We have not discovered heavy (mass>4Gev/c^2) charged particles reported by P. C. M. Yock. The upper limit of the intensity observed is 8.6x10^<-9>. We have upgraded the mass spectrometer for large acceptance and rejection of noises. one more layer of scintillator was added above the absorber, the number of scintillator in a layer was three times larger. The acceptance of upgraded spectrometer is 2297.5cm^2・sr, eight times larger. The muonーobservation without absorber gave the beta resolution of 5.5%, which greatly improved. The proton intensities were measured accurately in the beta range between 0.4 and 0.8. The intensities observed are higher than the previous observation and agree with the Sydney and Auckland's results. We are now observing heavy cosmic ray with iron absorber using large mass spectrometer. Less