- Detector -

How it works

The ISOMAX detector, which was an order-of-magnitude evolutionary step up from its progenitor IMAX, is a balloon-borne mass-spectrometer, that was launched for the first time in summer 1998 to measure the abundances and energy spectra of the isotopes of light elements (Li through the CNO-group) in the crucial energy range around 1 GeV/nucleon. Mass- and Energy-resolution should be good enough such that even a one-day flight will significantly improve the statistics of known particles in that energy region (see Streitmatter et.al. 1996).

The instrument employs the following components to measures the mass, charge and energy of a particle:

• a time-of-flight system, consisting of three layers of scintillators at the top, middle and bottom of the whole instrument stack attached to photo-multiplier tubes. These measure velocity for moderate particle velocities and also provide the measurement of charge, |Z|.

• a system of drift-chambers, in which the track of the particle is measured. This track will be curved, since the chambers are sitting inside the strong magnetic field (.88 Tm field integral) of a super-conducting magnet. The curvature is determined by the particle rigidity, or momentum per-unit charge.

• a set of aerogel Cherenkov counters, which will be used to determine determine velocity near the speed of light. Proper choice of the index of refraction of the aerogel will set the upper end of the energy range in which the instrument can be used.

The mass of the particle is determined from the magnetic rigidity and velocity of the particle, and the kinetic energy then follows from the velocity.

A charged particle in a homogeneous magnetic field will follow a circular orbit of radius r, where r is given by the magnetic rigidity R of the particle (and hence its mass A, charge Z and velocity, beta) and the strength of the magnetic field as r = R/B = p/ZB = beta * gamma * A /BZ. With Z determined by the TOF-scintillators and beta measured either by the TOF system or the Cherenkov counters, this can be solved for A if the radius of curvature r is sufficiently well measured. This provides all information about the particle species (A, |Z| and the sign of Z) and since E=gamma * m, this also provides all the necessary spectral information.

For the short-duration flight of approximate one day, the expected statistics of high energy particles were too low to warrant a very low index of refraction (n) aerogel, therefore the system was equipped with an n=1.14 Cherenkov counter, which limited the useful energy range of the instrument to < 1.5 GeV/nucleon and the results will rest more heavily on the TOF system. For a longer duration flight attempted in 2000, an n=1.045 Cherenkov would have extended this range out to about 3 GeV/nucleon. Alternatively, the instrument could have been used to extend the range of measurement to heavier isotopes (5 < Z < 14) in the same energy range, and the isotopes of Hydrogen, Helium and even anti-protons to even higher energies.

References:

Streitmatter et. al, NASA proposal, SSC-4A, 1996