The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 19?7

Absolute measureme nt of radiation brightness of New SUBARU undulator
Satoshi HASHIMOTO, Masahito NIIBE and Ainosuke ANDO
Laboratory of Advanced Science and Technology for Industry (LASTI),
Himeji Institute of Technology (HIT),
2167 Shosha, Himeji, Hyogo, 671-22 JAPAN

Abstract
Absolute measurement of the, radiation spectrum
in soft X-ray region from New SUBARU undulator isplaned. The principle of the measurement is based onenergy analysis of photoelectrons emitted by a rare gas
which has well known photoionization cross section, using a hemispherical electron analyzer. In this paper theundulator profile, measurement procedure and expected
results are described.

1 Introduction

toelectron Spectroscopy(UPS) to analyze chemical information or surface band structure of samples by a detected energy distribution of photoelectrons. An advantage of hemispherical analyzer over CMA is that a hemispherical analyzer is insensitive to sample position, thatis, an electric lens can quickly and easily be focussed on
the sample without the need for careful sample positioning and set-up. And it is possible to measure an angular
distribution of emitted photoelectrons by a hemispherical analyzer, though the number of collected electrons issmall compared to CMA.

2 New SUBARU undulators

1.5 GeV electron storage ring "New SUBARU"[1] isunder construction in the SPring-8 site. The ring has
4 straight sections for insertion devices; two planar undulators, super conducting wiggler and optical klystron
FEL. One of the two undulators is a 10.8m long device
with 200 periods that provides high-brightness soft Xray radiation.
For both radiation users and accelerator scientists itis important to know the performance of the undulator
which should be verified by measuring the absolute valueof photon flux or brightness. The spectral distribution,
peak height and fractional bandwidth of radiation depend on the actual machine parameters such as a finite
beam emittance, a finite angular aperture of observation and K parameter. Several papers on the absolutemeasurement of the brightness of undulator radiationfrom VUV to X-ray have been published[2, 3, 4, 5, 6].
In the range of photon energies under about 1 ke V, thephotoionization method is often used because the crosssection of photoionization of a rare gas is well known[7]both theoletically and experimentally and is. relatively
large. By measuring the energy distribution of the photoelectrons the absolute intensity and the band widthof radiation can be evaluated. The idea of this method
was originally described by Sasaki et al. [2] and applied
to the absolute measurement of undulator brightness at
SOR-RING[3] and PF[4, 5] by Maezawa et al. In thistechnique they used cylindrical mirror analyzer( CMA)
to measure the energy of photoelectrons. In contrast,
the higher energy region that photon energy exceeds 1keV suffers a rapid decrease of the photoionization crosssection of a rare gas. ill this energy range, X-rays scattered by He gas is measured using energy dispersive solidstate Si detector to evaluate radiation profile[6].
In New SUBARU the photon energy from the undulators is under 1 keV. Thus the photoelectron counting technique would be possible and we are planning
to measure the radiation intensity in an absolute scaleby a hemispherical electron analyzer instead of CMA.
A hemispherical analyzer is frequently used in X-ray
Photoelectron Spectroscopy(XPS) and Ultraviolet Pho--435-

The main parameters of New SUBARU storage ringare shown in Table 1. Two undulators whose total
lengths are 10.8m and 1.4 m will be installed in thestaright sections. The first experiment of brightness
measurement will be performed using the long undulator. The main parameters of the undulator are summarized in Table 2. It will be inserted into one of two 14m
long straight sections of the ring.
Table 1Main parameters of New SUBARU ringBeam energy
Current
Natural emittance
Betatron tune
Circumference
Energy spread
Life time

1.5 GeV
500 rnA677r nm-rad.
X:6.21 / Y:2.17
118.716 m
0.072 %
10 hourTable 2Main parameters of New SUBARU long undulator
54 mm
Wavelength
200
Number of period
10.8 mTotal length
25 - 44.5 mm
Gap length
0.49 - 0.16 T
Field strength
2.5 - 0.8
K parameter
Wavelength of radiation 13- 4.1 nm
The radiation profile of the long undulator are summarized here. The intrinsic half-angular divergence ofthe undulator radiation emitted by a single electron, Bn,
is given by
1 1 + K 2 /2(1)On=2nN'Y
where n is the harmonic number of radiation and N is
the number of periods of the undulator. In our case theThe 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

half-angular divergence of the fundamental radiation, e,'is 0.034 mrad for K =2.5.
The fractional bandwidth of the nth harmonic for asingle electron, !l.>.n/ An, is given by
1 2[ ( 1 ) -2'Y 2 ()2obs ] 1
A:-,....., nN + 1 +K2j2fl.>. n1.74 x 10 11 · E 2 [GeV]· N 2 · I[rnA.]

n2K2[x(1+K2/2)2 Jnt' (0-Jn2' (~)

Undulator(2)

where Bobs is the half-angular acceptance of obserbation. Assuming Babs=0.04 mrad, it is found that!l.>.d >. 1 =0.058 for the fundament! at K = 2.5.
The peak brightness of the nth harmonic undulator
radiation for the case of the zero emittance and infinitesimal aperture is given byBn

Monitorchamber)2(photonsjs · rnrad2 · 0.1%b.w.),(3)

where E is the electron energy in GeV, I is beam current,
Jm is the mth order Bessel function and ~ = ;~2n;;2 •The calculated peak brightness reaches to 4 x 10 18 at itsmaximum.
When the finite aperture of observation and the finite beam emittance are taken into account, the brightness is reduced as

Fig. 1 Schematic drawing of the undulator beamline
for absolute measurement of radiation intensity
h.v photoionization of a rare gas.
filter and passes only those electron having a specificmergy. Electrons entering the channeltron strike thechanneltron wall and cause secondary emission, accelerating along the wall of the tube, the r~peated collisions.have an avalanche effect. Thus a single electron input
gives risP to an output pulse of up to a hundred electrons. The analytical information is then derived frommeasurements of the energy distributions of the emitted
PlPctrons. The kinetic energy of the emitted electron Eke
is given by(5)Eke= hv- ¢,where hv is photon energy and ¢ is a threshold energyof ionizing an atom from its ground state to the ion.
Therefore, once the energy distribution of emitted photoelectrons is measured, the relative spectrum of incident radiation could be known.

(4)
Hemispherical /"_.-- -"""'-,In New SUBARU the beam divergence a~'< and a~ are{).09 and 0.03 mrad, respectively. So the effective brightness will be reduced to Bef 1 = 1 x 10 17 in our case. Thisvalue is about four order higher than PF undulator measured by Maezawa[4].
3 Absolute measurement of brightness::::: L~~d~~)J~4>

Undulator
I f I I I I I :J

Photoelectrons IChanneltron========---=( ..!.Wt RadiatitlnI U1 111::1Ionization chamber
10-20 m

3.1 Measurement procedureFig, 1 shows a schematic drawing of the beamline
set-up for the measurement. A differential pumping will
be used to separate the ionization chamber operated in
the 10-4 ,....., 10-5 Torr range from the high vacuum of thestorage ring. A deflection mirror will be used to avoid'"(-ray background which is generated by collisions of thestored electrons with the nucleus of residual gas atoms
or ions trapped in the orbit of the electrons.
Fig.2 shows the principle of the measurement. Inthe monitor chamber a gas cell is placed on the axisof radiation beam. The rare gas such as He and Ar isloaded into the cell and maintained at a constant pressure about 10- 4 ,....., 10- 5 Torr. Irradation of the rare gas
with soft X-rays ejects photoelectrons from atoms. Someelectrons generated in the cell go through an apertureon the cell and are detected by the hemispherical analyzer. The spherical analyzer acts as a narrow bandpass-436-Fig. 2 Schematic drawing of the absolute measurement of undulator brightness by use of a hemispherical electron analyzer.
The bandwidth of the fundamental radiation,!l.E / E, is 0.058. This -corresponds to !l.E=5.5 eV for
K =2.5. We will use VG 100AX of VG MICROTEH Co.
as the hemispherical analyzer, whose energy resolution
is about 0.05% and enough to measure the bandwidth.
The linear polarization of radiation from a planar undulator causes asymmetric angular distributionof emitted photoelectrons. The angular distribution isgiven by2a [1 + -(3cos{3I(B) =----:(} -1) ] ,47r2(6)where a is photoionization cross section, (} is the angle
between the electric vector of radiation and the direc-

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

tion of emitted photoelectron, (3 is called asymetric parameter and -1 < (3 < 2. The measurement will be
performed at the angle e = 0°' 90° or at the magic angle = 54.7° where the effect of asymetric parameter
vanishes.
For absolute measurement of radiation intensity thecounting efficency of the device must be calibrated. Thatis, it is nessesary to know the relation between the number of incident photons and the number of electrons
counted by the channeltron. The total number of photoelectrons generated in the ionization volume per second,
dN1 , can be calculated from the incident photon number
with photoionization cross section[7] and the number ofatoms of a rare gas in the ionization volumee(7)where CJ is photoionization cross section, nn density ofatoms, £ the length of ionization volume and I is theintensity of radiation in photons/sec. The number ofphotoelecton injected to the electric lens of the analyzer,
dN2 , is derived from the total number of photoelectron(8)

where f is a factor which depends on both angular distribution of photoelectrons and a solid angle that sees
the aperture of the electrostatic lens. The number ofelectrons counted by the channeltron, dN3 , depends on
the number of injected photoelectron, dN2 , and several
parameters of the analyzer such as channeltron voltagev(9)

where ((V) is an amplification factor of the channeltron
which depends on the channeltron voltage V, TJ is thetotal efficiency of the measurement. Therefore, once thetotal counting efficiency of the device, TJ, is calibrated,
the absolute brightness of the radiation can be evaluatedfrom the counted number of electrons.
For calibration of the device, we will use an electon
gun whose beam properties such as current and beam
size are known and investigate the relation between thenumber of electrons form the gun and the counted number of electons amplified in the channeltron under thesame experimental conditions as the actual measurement. According to this calibration data, we can estimate the brightness of undulator radiation from thenumber of counted electrons. The efficiency could beincreased by an optimization of the dimension of cell,
an increase of the pressure of rare gas and an suitablechoice of the kind of rare gas whose cross section is large
at the desired photon energy region.

3.2 Expected results
Here we can estimate the number of electrons which
can be counted by the channeltron in the case of New
SUBARU long undulator. Assuming that £=1cm, nn =1.6 x 10 12 , K =2.5, radiation wavelength 10nm and He asa rare gas, the total number of photoelectrons generated
in the cell, dNt, is about 750 electrons/sec. Assuming
the isotropic distribution of emitted photoelectrons for-437-

simplicity, the number of photoelectron injected to theanalyzer, dN2 , is about 10 per seccond in 0.1%bandwidth. Thus the counting rate in the channeltron is expected to be about 1000 electn~ons per sec. For precious
estimation of the number of photoelectrons, both theprecious measurement of the pressure of rare gas in thecell and the precious calibration of TJ would be key-issues
in this method.
4 Summary
Measuring the photoelectron emitted from rare gasatoms with the hemispherical electron analyzer, the radiation profile of the New SUBARU undulator in soft
X-rays will be evaluated. The energy distribution ofemitted photoelectrons gives the bandwidth of radiation. And when the device is well calibrated, the brightness can be evaluated in an absolute scale. The precious measurement of the density of rare gas and theprecious calibration of the device would be important.
The counting ratio could be increased by optimazationof the device and set-up parameters. The experimentwill be performed after the install of the long undulator.
Acknowledgement
The authors would like to thank Prof. Koyano ofHimeji Institute of Technology for his useful suggestions.
One of the authors (S.H.) thanks to Dr. Minagawa of
KEK for his advice on UPS study.
References[1] A. Ando et al.,"VUV and SOFT X-RAY LIGHT
SOURCE "New SUBARU" ", PAC97 (1997)[2] T. Sasaki et al., "Measurement of absolute intensity of radiation in XUV by a rare-gas photoelectron
counter", Appl. Opt. vol.16 (1977) 3115
[3] H. Maezawa et al., "Absolute measurement of undulator radiation in the extreme ultraviolet", NIM 208(1983) 151.
[4] H. Maezawa et al., "Spectral characterization of undulator radiation in soft X-ray region", Applied Optics vol. 25 (1986) 3260
[5] H. Maezawa et al., "Characterization of undulator
radiation at the Photon Factory", NIM A246 (1986)
82[6] U. Hahn et al., "Measurements of emittance andabsolute spectral flux of the PETRA undulator at
DESY Hamburg", J. Synchrotron Rad. 4 (1997) 1[7] G.V.Marr et al., "Absolute photoionization crosssection tables for Helium, Neon, Argon and Kryptonin VUV spectral regions", Atomic Data and Nuclear
Data Tables 18 (1976) 497,