Gaseous AntiParticle Spectrometer (GAPS)


1. Introduction

    We  propose a  novel  antimatter detection  scheme. The gaseous  antiparticle spectrometer (GAPS) effects particle identification through the characteristic X-rays emitted by antiparticle when it forms exotic atoms in gases. The resulting instrument can provide more than an order of magnitude improvement in sensitivity over the forthcoming missions in the next decade. GAPS will draw  several significant cosmological consequences, most prominently the indirect detection of dark matter in the Galactic halo through measuring low energy antideuterons. In addition, GAPS will elucidate a source of antiprotons and significantly extend a searching power of antihelium to probe the baryonic symmetry of the Universe. Based on the full understanding of relevant physics, we have  simulated several  potential applications of  GAPS for  measurement of different types of cosmic antiparticles.

2. Science of GAPS

    GAPS is tuned to detect cosmic antibaryons at low energy band (< fewx100 MeV/nuc).

Antiparticle Antiproton Antideuteron Antihelium 
Primary source WIMP annihilation WIMP annihilation Antimatter domain in the universe
Secondary source Spallation Spallation None
Previous measurements Detected at E=0.2-20 GeV No detection No detection
Scientific study Solar modulation
Dark matter Baryonic (a)symmetry


3. Detection scheme of GAPS

    The GAPS detector operates on a straightforward scheme. An antiparticle that passes through a TOF system is slowed down by dE/dx losses in a degrader block. The  thickness of this  block is tuned  to select the  sensitive energy range of the detector. The antiparticle is eventually stopped in the gas chamber, forming an  exotic atom with probability of order unity (excluding a small fraction of loss by the direct annihilation in degrader block and gas). Once the antiparticle is captured into an bound state of the atom due to its negative  charge, it starts decaying toward the nucleus. Through proper selection of gas and its density, the absorption of the antiparticle can be tailored to produce 3 to 4 well-defined hard X-ray transitions in the decay chain. Coincidence of deexcitation X-rays unique to antiparticle type register a valid event  and the particle type is  identified through energy measurement of characteristic X-rays. Incident energy is reconstructed from the velocity measurement by TOF system. Promptly after the release of these X-rays, the antiparticle annihilates with the proton producing a shower of pions. The coincident signal between the TOF system, the characteristic decay X-rays and the energy deposition induced by the pions is an extremely clean criterion for the presence of the antiparticle.

    Each surface of GAPS detector is readily open to half of the sky due to the excellent background rejection power. Therefore, the effective grasp of GAPS is significantly larger than a detector of same size based on the other detection schemes. Also, by varying the thickness of the degrader block, separate energy channels  can  be defined and optimized. A typical GAPS implementation would have 2 to 5 such channels. In principle, there is no inherent lower limit on the detectable energy of cosmic antiparticles. Instead, it is set by the geomagnetic rigidity and the particle degradation by the materials (e.g. TOF system) outside the absorbing material.

    Characterisic photons in hard X-ray band  are chosen in order to avo

4.Comparison with other detector schemes

Detector scheme GAPS Magnetospectrometer Calorimeter
Particle identification Characteristic X-rays Momentum + velocity Total deposited energy
Geometrical grasp Large Small Small
Quantum efficiency Medium  Medium high
Effective grasp Large Medium Large
Total energy band width ~400 MeV/nuc ~10 GeV/nuc ~200 MeV/nuc
Lower energy limit ~ 50 MeV/nuc ~200 MeV/nuc ~ 50 MeV/nuc
BKG rejection power Excellent Medium Poor
Background source Cosmic hard X-rays Multi-scattering  Protons
Identifiable particles Antibaryons All cosmic-rays Antiproton


5. Potential mission of GAPS

Mission type Balloon Space mission Interplanetary mission
Purpose Spectroscopy of low energy antiprotons Search for antideuterons and antiheliums Spectroscopy of  very low energy antiprotons
Scientific objectives Low energy antiprotons,
Solar modulation
Dark matter,
Baryon (a)symmety
Primary antiproton sources, Solar modulation
Observation point High latitude
(Canada, North poles)
Low earth orbit 
(High latitude)
Beyond solar system
Size (cube) 2 m 5 m 6 cm
Weight < 1 ton < 5 ton ~ 1 kg
Energy band 100-400 MeV/nuc 100-400 MeV/nuc  < 100 MeV
Observation time 1 day 3 years 1 year

6. Current status

7. Collaborators

7. Publications and talks

Last updated on 12/07/03.