CREAM

Home page of the Experiment: http://www.unisi.it/fisica/cream

CREAM (Cosmic Ray Energetics And Mass) belongs to a new generation of balloon-borne instruments designed to provide direct measurements of the individual energy spectra of primary cosmic rays (from proton to iron) and of their elemental composition at energies approaching 1015 eV. The main science goals of the experiment are to shed light on the so far unknown mechanism of acceleration of primary cosmic rays of very high energy and to improve the understanding of their interactions with the inter-galactic medium.

CREAM will have adequate exposure to resolve the shapes of the cosmic-ray H, He, and heavier nuclei energy spectra at energies approaching the lower end of the transition region of the spectrum, known as "knee", where the all-particle spectral index increases from ~ 2.7 below 1014 eV to approximately 3.3 above 1016 eV. Taking advantage of the new Ultra Long Duration Balloon (ULDB) flight capability under development by NASA, CREAM will be able to reach 500 TeV after a series of 3 flights designed to last from 60 to 100 days each. The possibility to explore spectral features beyond 100 TeV will allow CREAM to search for the onset of cutoffs in the individual spectra of light elements at an energy scale of order Z x 1014 eV, where Z is the charge of the primary nuclei, as predicted by a class of models of CR acceleration by supernovae shocks.

CREAM is under preparation for its first balloon flight. According to the current schedule, the first launch will take place from Antarctica in December 2004.

A key feature of the instrument is that the measurement of the energy of the incoming particle relies on two complementary techniques based respectively on a 20 X0 sampling Tungsten/Scintillating fiber calorimeter preceded by a densified graphite target and on a Transition Radiation detector (TRD). Simultaneous measurements of the energy and charge of a subset of nuclei, by the calorimeter and the TRD, will allow in-flight inter-calibration of their energy scales. The primary cosmic nucleus interacts inelastically in the target and generates secondaries, mostly charged and neutral pions. The latter initiate an e.m. shower whose energy and direction is reconstructed by the finely segmented imaging calorimeter. The reconstructed axis of the shower is projected backwards and matched with the SCD pixels to provide an unambiguous measurement of the charge of the primary by discriminating against those pixels affected by backscattering from the calorimeter.

The ability to identify the incoming primary particle charge relies on multiple accurate measurements of its charge. From top to bottom, charge is measured by a segmented timing-based charge detector (TCD), a pixelated Silicon detector (SCD) and up to six planes of scintillating fibers (S0,S1,S2 hodoscopes) providing dE/dx measurements and track-reconstruction capability.