Measuring the NSRL Beam Energy using Time of Flight

One method of measuring the kinetic energy of beam particles is through their time of flight between two detectors.  The detectors used are plastic scintillators mounted on photomultiplier tubes to register the light given off when a charged particle passes through.  The upstream detector starts a clock, and the downstream detector stops the clock.  Knowing the distance between the two detectors allows the velocity of the particle to be determined.  In practice, one detector (Clock Start) is kept in a fixed location while the second detector (Clock Stop) is located at an upstream and downstream location with a well determined separation.  The change in the Clock Stop times is used to determine the time of flight.

Relativistically, as the particle velocity approaches the speed of light, the sensitivity of the time of flight method decreases.  For NSRL applications, the greatest sensitivity is for particles in the kinetic energy range between 100 MeV/n and 500 MeV/n, although measurement is possible up to kinetic energies of ~1000 MeV/n. 

Figure 1: The timing distribution of the clock start and stop pulses used to determine the time of flight. 

Since the Clock Start pulse starts the clock, its time distribution is very narrow and reflects the intrinsic time resolution of the system.  In Fig. 1 the horizontal axis is in units of "clock ticks" which correspond to 35 picoseconds per tick.  The width of the start pulse indicates that the time resolution is on the order of 25 ps.  The Clock Stop pulse width is governed by several factors including the momentum spread of the beam particles, and the amount of light generated by the particles in the scintillator. 

This technique was used to measure the beam energy of proton beams with nominal kinetic energies of 200, 500 and 1000 MeV.  Table 1 shows the results of these measurements.

Beam Energy from RF Bragg Peak Energy Time of Flight Energy
200 MeV 205 MeV 205 MeV 2 MeV
500 MeV 506 MeV   509 MeV 10 MeV
1007 MeV No Bragg result possible 1006 MeV 26 MeV

For a detailed discussion of Time of Flight measurements at NSRL, please see the NSRL Technical Notes NSRL-TN-002-05 and NSRL-TN-001-06.

 

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Last Modified: February 1, 2008
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