Alpha Magnetic Spectrometer Animation

The AMS spectrometer reacts in a predictable way to the passage of a charged particle. This simulator illustrates the behavior over the expected range of inputs. The sought particles are nuclei of atoms, both normal and anti-matter. Particles less than 1 MeV are stopped by the shield. Heavy fast particles are stiffer and therefore show less curvature of track within the magnetic field of the spectrometer.

The two diagrams show the detector in two views. The upper diagram shows a side view of the components of the detector as a vertical slice. The parts of the detector in this diagram have pop-up names (use the mouse). The incoming particle from the zenith (top of the diagram) first encounters the low energy particle shield, then passes the first time of flight scintillators (green), then the trackers (red), then the second time of flight scintillators (green) and finally the cerenkov counters (brown). Particles entering the detector at an angle may impact the anti-coincidence scintillators (blue) and magnet itself (yellow).

The lower diagram shows the circular top of the magnet. A particle must impact the detector somewhere in this area to interact with the active elements.

The direction of the particle and its point of impact on the detector surface determine its track through the active components. The direction is set directly in the animation with the mouse. Both the x-y position on the circular top of the magnet and the left-right position of the incoming particle above the spectrometer diagram respond to mouse actions.


A moving charged particle is deflected by a magnetic field. In a constant field, the track will have a constant radius of curvature in the plane perpendicular to the field. The effective track is a helix with axis aligned with the field.

Assume the track is in a vacuum.
symbol description initial
H magnetic field (vector) 1.5K gauss
v velocity (vector) 0.8 speed of light
q charge (multiple of proton charge) a*1.6021917E-19 coloumb
m mass (positive multiple of proton mass) b*1.66044E-24 gm
c speed of light 2.99792458E10 cm/s
gamma 1/sqrt(1-v^2/c^2) 1.666
w (q*H)/(m*gamma) 8.68E7*a/b
Z angle between v and H .896 radians
r radius of curvature, v sin(Z)/w 216 b/a cm

Detailed information can be found on the AMS homepage.