The 9th Symposium on Accelerator &ience and Technology, Tsukuba, Japan 1993

A Characteristic of Photon Stimulated Gas Desorption from Aluminum
Nobuhiko Ota*
The Graduate University for Advanced Studies
Ken'ichi Kanazawa, Yusuke Suetsugu, Masanori Kobayashi, and Hajime Ishimaru
National Laboratory for High Energy Physics
1-1 Oho, Tsukuba, Ibaraki, 305 Japan
In this paper, we removed carbon
on the surface of several aluminum
samples by argon (Ar) ion etching andmeasured thephotodesorption yield (1J ) of carbonategases from these samples. Quantitative
relation between the 1J and the carbon on
the surface was obtained and the source ofdes orbed gas was suggested. The results
are detailed in the following.

Abstract

The photon stimulated gas
desorption (PSD) from aluminum isinvestigated experimentally in relation to
the number of carbon on surface. The
photodesorption yield (1J ) during the firststage of irradiation reduced by threeorders of magnitude by removing thecarbon on surface using argon ion etching.
The amount of desorbed CH4, CO andC 0 2 decreases almost linearly as
increasing the quantity of carbon
removed. The results indicate that
desorbed gas comes from the surface layerof aluminum in the initial stage ofirradiation at least.

II. EXPERIMENTS
The experiment was performed at
NE9 beam line of TRISTAN
Accumulation Ring. The beam energy was
6.5 Ge V and the critical energy of SR was
26.3 ke V. The SR was collimated to 10mm x 10 mm at 15 m from the source
point. Incident photon number per beam
current per second was 5.3x101 8
photons/sec/rnA. The collimated photons
entered a sample chamber through an
orifice with an aperture of 20 mm x 20mm (conductance of 461/s for N2 at roomtemperature) and hit normally aluminum
samples. Total pressure of both side oforifice were measured using extractor
gauges (LEYBOLD) calibrated previously
by a spinning rotor gauge. Partial pressurein the sample chamber was measured
using a quadrupole mass spectrometer(Balzers). The 17 is calculated byI. INTRODUCTION
The gas desorption from beamchambers irradiated by synchrotron
radiation (SR) emitted from electron or
positron beams is called the photon
stimulated gas desorption (PSD). The PSD
is very important on accelerator operation,
since it reduces beam lifetime or increases
the beam emittance. Lots of studies on thePSD have been reported and somecharacteristics have been made clear. Gas
components desorbed from common beamchamber materials, such as stainless steel,aluminum and copper, are mainlyhydrogen (H2), methane (CH4), carbon
monoxide (CO) and carbon dioxide(C02). However, the quantitative relation
between the condition of the irradiatedsurface and the amount of PSD, which isessential to understand the mechanism ofPSD, has not been made clear.

11 = K X C X { ( ~ - P~) -( Pt - ~)}Np'.................... (1)where K is a constant = 3.3x101 9[molecules/Torr/l], C [l/s] is theconductance of orifice for each gas species
and Np [photons/s] is the number ofphotons. P 1 and P2 [Torr] are the*Yaskawa Electric Corporation, 2-1 Kurosaki,
Yahatanishi, Kitakyushu, 806 Japan.

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pressures in the sample chamber and beamline, respectively. The superscript SR andb above Pi and P2 means the pressuresduring and before photon irradiation,respectively.
The sample chamber has an iongun to remove carbon on aluminum
samples. The surface analysis chamberwith an Auger electron spectrometer(AES) is connected through a gate valve tothe sample chamber. The surface analysis
of irradiated sample pieces, therefore, canbe done without exposing them to air andat any time.
Sample pieces were cut from EXextruded aluminum alloy (A6063-EX) [1]by oil-free machining. They received anultrasonic cleaning in an acetone bath. The
area of sample piece has 27 mm x 30 mm
with a thickness of 3 mm. These pieces
were etched by argon ion in different ion
dose and analyzed by AES. The amount ofcarbon removed was calculated using
depth profile.

shown in Figures 2 and 3, respectively.
The 1J decreased in the order of thenumber of carbon removed. The results
indicated the strong correlation between 1J
and the number of carbon remaining on
the surface.
The 1J of CI-J4, CO and C02 at
the photon dose of lxlo20 photons/cm2
became almost equal for all samples as

Figure 1. Total photodesorption yield.

III. RESULTS AND DISCUSSIONS
Four sample pieces, SP1 - SP4,
with different quantity of carbon wereprepared. Argon ion dose and total numberof removed carbon for each sample werepresented in Table 1. Figure 1 shows thetotal 17 in N2 equivalent for each sample
as a function of photon dose. The 17 at thephoton dose of 2x1o17 photons/cm2 ofSP3 and SP4 were lower by one order ofmagnitude th~m that of SP1 (untreated).
The 17 of SP2 was further lower by about
two orders than SP3 and SP4 at the same
photon dose. The 17 for CO and C02 are_10- 2oo"'C ........

SP f
OCJX:Ic::·-0---------~~---------------+----------------1---------------

c:0SPJ :•Q)

3>- ....... 10-o-g,::; .......
c.tn

10I..
oQ)

tnQ)

4-c2
0....... 0 10- 5
0 E..c: '-'a.
1 0- 6\o~~"'sL
Li ·SP4::;:,ui

llp~~~i

1: • • l: ·l~B,~
Jsl I''N.

----M·&r.~------------·-·t···-·a··-&-~-oi·-~t--------·.. SP2i••10 17•! •1.

.

.

10 I 81 0 19

1 020

1 021

Photon dose (photons/cm 2 >Figure 2. Photodesorption yield for CO.

Table 1 Arion dose and total numberof removed carbon
Arion dose Removed carbon
ions/cm2
atoms/cm2SP100
2.2xiOl~
3.7xiol6
SP2
2.2xiol7
1.1xiol6
SP3
5.7xloi7
1.8xiol6
SP4

shown in Figures 2 and 3. Total amountsof these gas species integrated u·p tolxlo20 photons/cm2 for each sample areshown in Figure 4 as a function of theamount of carbon removed. The reductionof the total amount of main carbonate gasspecies is apparently proportional to theamount of carbon removed. A dashed line
in Figure 4 was obtained by a least square-262-

60 IN~ u0+E-0Q)u..c+...,

E:r::~u-c (f)Q)~-o-~OlINEu.........

U'l4~

~

~30E~"'0E.. Q)

Ol..c00XQ)-'-=

5~PJj _______ --·~i.. __'\_·--·~----·--···----!-~:i" isp 4 ii "-----------·<•-··-·------l-··-·····--·-->-----·---··-

SP3!

:·~:!

"!

Isr2~

2-------·----j··········-·--:-----";--·---t·-----------

1____________ L_.

:

E..

U'lQ)

"'0

--!------·-·---~-\,. _____ _

0
0
1
2
34
Integrated number ofcarbon removed by etching( x 10 16 atoms/cm 2 )method and its slope was about -1. Thissuggests that the gas desorbed comes fromthe surface layer of aluminum in the initialstage of irradiation at least.

Figure 4. Total amounts of carbonate gas species
integrated as a function of the amount of carbon
removed. A dashed line in figure is obtained by aleast square method and its slope is about -1.

IV. SUMMARY AND CONCLUSIONS

number of carbon removed. It wassuggested that the source of desorbed gas
in the initial stage was the surface layer ofaluminum.

The relation between the 17 ofcarbonate gases from aluminum and thenumber of carbon on the surface layer wasstudied. The 11 at the initial photon dose
reduced drastically by removing carbon onaluminum surface by ion etching. The
amount of desorbed CH4, CO and C02
decreased almost linearly as increasing theV. REFERENCE[1] H. Ishimaru, J. Vac. Sci. Technol.

A9, 1984, pp. 1170.

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