The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

Beam energy measuremen t by the time-of-flight technique
Susumu OKUMURA, Kazuo ARAKAWA, Mitsubiro FUKUDA, Ikuo ISHIBORI,Hiroyuki TAMURA, Takasbi AGEMATSU, Takayuki NARA, Watalu YOKOTA
Japan Atomic Energy Research Institute (JAERI)
1233 Watanuki-machi, Takasaki, Gumna, 370-1292, Japanequation:
Abstract
The absolute energy of ion beams accelerated by theJAERI AVF cyclotron has been measured by the time-offlight technique. We have developed a movable TOF
counter system for precise energy measurement, which canbe achieved by two TOF measurements for different flightpath length.
1 Introduction
Beam energy is an important parameter for experimentsof research of irradiation effects. The nominal beamenergies of the cyclotron are based on the ones from which
initial parameters of the cyclotron tuning are calculated.
Actual beam energies are not exactly the same as thenominal ones since the parameters usually have to bechanged for optimum tuning of the cyclotron. The energy ofthe beam extracted from the cyclotron can be estimated withan analyzing magnet while the precision of the energy
measurement is not so high for uncertainties of the beam
path and the magnetic field in the magnet. A crossover
technique 1•2> is a good method to measure an absoluteenergy for protons with energy below 50MeV. However, it
is useful neither for above SOMeV protons due to very small
elastic scattering angles nor for heavy ions due to difficultyof finding appropriated targets. A time-of-flight method(TOF) 3> is easier way to determine the absolute energy forthe variety of ion species with high precision better than theother energy measurement method such as the crossover
technique and analyzing magnet.
The kinetic energy of an ion can be obtained from thev{3=cwhere 111o is the rest mass, c the speed of light, and v thespeed of an ion. The speed v can be obtained· frommeasurement of the flight time and the flight path length.
The uncertainty in the kinetic energy can be evaluated fromthe uncertainties in the flight time and the path length, since
the contributions of the rest mass and the speed of light arenegligible. Because of the short flight time, 10 - 40 ns/m,
the flight time should be determined with accuracy of theorder of 10 ps to obtain dElE with an accuracy less than 1o·3 •It is difficult to measure the absolute flight time, whichneeds a common time origin for ion detection at two
different points, with such a good precision. Therefore, wehave developed a movable TOF counter system in which wedon't need to know the time origin, as shown in Fig.l.
2 Beam Energy Measurement System

2.1 Start and stop counter

The TOF system has two ion detectors: one is a start
counter for production of a start signal and the other, a stopcounter, for production of a stop signal.
A beam phase monitor4l, which has been developed tomonitor timing characteristics of the beam without beam
degradation, is used as the start counter. The beam phase
monitor is a fast timing detector with a micro-channel plate
which is multiplying secondary electrons coming from astop counter

start countermagnetic scalebeambellows
pulse motor

Fig. 1 Layout of the movable TOF counter system.

-471-The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

thin target foil, a 3 micron thick aluminum-strip, when ionspass through it. The foil is mounted at a distance of 50 mm
from the MCP. The best time resolutions are 190 ps FWHM
for 225 MeV 160 7+ and 230 ps FWHM for 220 MeV 12C5+.
A MCP was used as the stop counter, which can directly
detect ions and produce a fast timing signal.

] 2000-------~---------:------- ;._;:-----+- ---j:: !ii~Fq::

2.2 TOF systemo .................._ ......

~...!d_~Lo___,__.__,___c,_....J..,~.J................JThe velocity of the ions can be evaluated from thelength between the detectors and the time difference of thestart and stop signals. Using mass references, the energy of
the ion can be calculated from the velocity. The precision of
the energy measurement mainly depends on theuncertainties of the measurements of the length and the timedifference. Since the maximum velocity of the ion
accelerated by the cyclotron, which is obtained in the caseof proton, is 0.41 of the light velocity, precise measurementof the length and the time difference is needed for precise
energy measurement using the time-of-flight technique.
Errors in absolute measurement of the length and thetime difference mainly depend on inaccuracy of decision of
the criteria, such as time zero for the measurement of thetime difference. In an actual measurement system, thesignals from the start and the stop counters have different
time delay caused by the dilfference of cable length andtiming properties of the counters.
To achieve precise measurement of the beam energy, weadopted relative measurement of the length and the timedifference, which eliminates inaccuracy of decision of thecrite1ia. In this measurement, the length and the timedifference have to be measured twice by moving the stopcounter.
The counters are installed in different vacuum chambersconnected with bellows. The chamber for the stop countercan be moved at the maximum distance of 2 m. The relativechange of the length can be measured with a magnetic scalewith a precision less than 0.1 mm. The relative change ofthe time difference is obtained as a shift of the time
spectrum peak measured with a time-to-amplitude converter
using the relative timing signals from the start and stopcounters.

100 150 200 250

300 350 400 450 500

ChannelFig.2 A time-of-flight spectrum for Ar
ions of the cocktail beam.
(mass/charge=5), as shown in Table 1. Since the energy loss
in the foil of the start counter is large for heavy ion beams, a1011 g/cm 2 thick carbon foil was used for the 220MeV(nominal) 12C5 +, and the Cocktail beam. In the case of thecocktail beam, a semiconductor detector or a plastic
scintillation counter, which is not superior to the MCP in
timing characteristics, was used as the stop counter toidentify ion species from the energy spectra.
Figure 2 shows a time-of-flight spectrum for Ar ions of
the cocktail beam with a FWHM width of 2.3 ns at theminimum flight length. The flight time can be evaluatedfrom difference of the peak positions at different flight path
length. The accuracy of the energy measurements, which
was around 0.5 %in theses measurements, can be improvedby more accurate time calibration.
References[l] B. M. Bardin and M. E. Rickey, Rev. Sci. Instr. 35(1964) 902-903.
[2] S.Okumura et. al., JAERI TIARA Annual Report 1993
(vol.3), 202-204.
[3] K. Neldner et al., Nucl. Instr. and Meth. A274 (1989)
419-424.
[4] S. Okumura et al., JAERI TIARA Annual Report 1996,
JAERI-Review 97-015,220-221.
[5] M. Fukuda et al., The ll th Sympo. on Accel. Sci. andTech., Harima, Japan (1997) 139-141.

3 Measurements and Results
We measured the beam energies of a IOMeV (nominal)
H+, a 220MeV (nominal) 12C5+, and a Cocktail beams5)

Table 1 Result of beam energy measurement.
Ion
H+12cs+Cocktail(M/0=5)
IsN3+z~e4+

4oArs+
84Krl7+

Nominal energy (MeV)
10

Measured energy (MeV)

220

225.2+0.6

56

57.2+0.1

75

76.4+0.2

10.17+0.02

150

153.2+0.3

315

328.6+0.7-472-