T-535: Antiproton Beam Testing of the Gaseous Antiparticle Spectrometer (GAPS)
The goal of this experiment was to test a novel concept for space-based antimatter detection.
When antimatter is slowed down in a target it is captured and forms an exotic atom. The exotic
atom deexcites and in the process emits X-rays. When the antiproton reaches the ground state it
annihilates with the nucleus producing pions. The signature of an antiparticle is multiple X-raysof precisely defined energies and one or more pions, all occurring within a sub-10 nanonsecondtime window. GAPS great improvement in sensitivity over the current premier techniques for
antimatter searches, such as the BESS experiment, which uses a superconducting magnet. Assuch GAPS is a next generation concept for antimatter detection.
S1-S5

P1

V0
V1-V6

BeamTarget

Detector
P0P2

Degrader

CER P3

P5

Figure 1: Experimental layout: The detectors highlighted in green are new for the 2005 experiment.

The experimental setup is in Figure 1. The GAPS gas target is designed so that antiprotons withkinetic energy below 100 MeV will be captured into exotic atoms with nearly unity probability.
The 1 GeV/c (433 MeV) antiprotons were slowed passively using a lead degrader. The TOF andenergy deposited in a third beam counter after the degrader is used to tag the antiprotons thatsurvive passage through the degrader and enter the GAPS target. A Cerenkov detector is also
used to veto highly relativistic charged particles – both in-flight antiproton annihilation products
and accidental beam particles (pion, kaon, muon and electrons). Together, these four countersefficiently tag antiprotons entering the GAPS target.
GAPS as employed at KEK in the pi2 beamline is shown in Figure 2. The gas target cell wasbuilt using carbon fiber reinforced plastic (CFRP). Sets of 2x4 NaI crystals (25mm diameter, 5mm thick) are housed in 16 panels arranged in a hexagonal array. Each of the 128 crystals iscoupled to a Hamamatsu RM1924a photomultiplier tube (PMT). The system achieves sufficientenergy resolution to resolve the X-ray transitions of interest and 200 ns time resolution forcoincidence rejection of background. An open ended cylindrical geometry is required for thebeam entrance and exit.

Figure 2: Photograph of the Gaps detector with the gas target in pi2 beamline

We measured the antiproton rate to be 27 antiprotons per spill. This rate was extracted from theGAPS data. Nearly 50 exotic atoms formed every hour of run time. A histogram of theantiparticle TOF is in Figure 3.
2500
Run 265
Center = 15.69 ns
Sigma = 0.38 ns
Back = 823.33
Triggers = 145208
Pbars = 19248
Time = 67 min
Spills = 951
PbarRate = 20.2 /spill

Counts per 0.05 ns2000

1500

1000

50001214161820
P2-P1 Time [ns]2224Figure 3: Histogram of the antiparticle TOF from P1 to P2. The blue Gaussian fit to the antiproton peakon top of accidental pions and muons generated from kaon decay

During the 31 hours in which we ran with the C2F6 target, approximately 1500 exotic atoms wereformed. A histogram of all X-rays detected in this C2F6 target dataset is plotted in Figure 4. The
X-ray spectrum with the target removed is also plotted (1.5 hours equivalent data scaled to theC2F6 data). A line fitting this data indicates a flat background with low energy component.

Figure 4: Histogram of X-rays for C2F6 target (31 hours). The X-ray spectrum with the target removed isalso plotted (1.5 hours equivalent data scaled to the C2F6 data0. A line fitting this data indicates a flat
background with low energy component.

In addition to the antiprotonic X-rays, we expect to see several charged and neutral pions (5 onaverage) emitted from this annihilation. The charged pions will leave an enormous amount ofenergy as they pass through the NaI crystal (~2 MeV) that will be recorded as a pion star event(π*). As an example of the power of the technique is shown in Figure 5. The cylindrical detector
has been unfolded to show the 16 NaI detector modules each containing 8 independent X-raydetectors. A GAPS event is shown with three X-rays and a pion star. This is considered a clean
signature of a p F atom since the three X-rays are consistent with expected transitions at 93, 43
and 23 keV and are observed together with a pion star. This is the first time a detector has everbeen made whose purpose is to detect antimatter through a coincident exotic atom signature. Ourdetection rates and efficiencies as inferred from experiment are consistent with simulations.

BeamDirection

33

!*38
86

Figure 5: Antiprotonic event in Fluorine indicating correct signature of pion star and 3 X-rays.

The GAPS experiment succeeded in completely confirming the validity of the exotic atom
approach to antimatter detection.