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AAASTER

;old Highly Ionized !ona: Comparison of Energies of Recoil IonsProduced by Heavy Ions and by Synchrotron Radiation X raya

1. A. Sellln, J. C. Levin, C.-S. 0. H. Cederqulat, S. B. Elston, ;!i. T. Short, and H. Schml dt-Bocki ng1

Department of Phy3lcs, University of Tennessee, Knoxvllle, TN 37916,and Phyalca Division, Oak Ridge National Laboratory, Oak Ridge, TN3783' . U. S. A.

Abatract

The energies of highly excited, high-charge-state recoi l Ions

produced by fast heavy-ion Impact on target atoms ("hammer" method)

have been compared with the energies of slmilai—charge-state recoil

Ions produced by vacancy cascades subsequent to Inner-shell photoab-

aorptlon of tuned synchrotron radiation x rays ("scalpel" method).

These comparisons show that the "hammer" method leads to recoil Ion

temperatures typical ly 4 orders of magnitude lower than those which

occur In plasma sources In which Ions of 3lmllar lonlzatlon and

excitation states have comparable abundance, while the "acalpel" method

leads to temperatures up to 6 orders of magnitude lower. Advantages

and drawbacks of each method for potential precision spectroscopy of

stored or trapped high charge state Ions, and for production of

extracted beams of low etnlttance for use In secondary Ion-atom

col l is ion studies at eV to keV energies are dlacusaed.

1. Introduction

In this paper we review recent progre33 In the quantitative

determination of recoi l Ion energies produced U3lng the "hammer" and

"scalpel" methods, emphasizing those aspect3 which have principal

applications to the central theme of this conference: access to the

physics of stored and trapped part icles. Generally, the colder the

species of Interest which are produced, be they neutral, singly, or

multiply charged systems, the better, since the potentiaL for

%o"BlSTBIBUTIOfl OF THIS SliOSJiVlENT IS UNLIMITED

r e f r ige ra t ion to s t i l l lowor temperatures la a centra l concern of many

s c i e n t i s t s in the f i e l d . Production of highly Ionized and excited ions

at low temperatures poses a particularly severe challenge, since

typical sources of such ions such as stellar, fusion, and laser plasmas

typical ly involve temperatures In the 1 to 100 keV temperature region

[ 1 ] , and fast beam sources which achieve similar lonlzatlon-excltatlon

s ta tes typically involve beam veloc i t ies v/c - 0.1 [ 2 ] . In both cases

Doppler spreads and sh i f t s tend to severely limit spectroscoplc

precision. Similarly, the emittance of plasma and fast beam sources i s

not attractive if one has the objective in mind of carrying out

high-charge-state ion-atom col l i s ion experiments [3] at eV to keV

energies under conditions where good energy and angular definition of

high-charge-state project i le tons are important. As wi l l be seen

below, the "hammer" method permits achievement of 1 orders of magnitude

lower temperature than plasma sources in which similar

ionizatlon-excltation s tates have comparable abundance, as well as 3

orders of magnitude advantage in v/c re lat ive to fast beam sources.

The "scalpel" method permits achievement of up to 6 orders of magnitude

lower temperature, and 1 orders of magnitude advantage in v /c . These

advantages are accompanied by the drawback of recoi l sources being

re la t ive ly weak sources, a disadvantage of l e s s importance in trapping

and storage devices than in many other applications.

The low recoil- ion energies Intrinsic to the "hammer" method were

f i r s t p o i n t e d o u t and v e r i f i e d by S e l l i n e t a l . [ H ] , an o b s e r v a t i o n

soon thereafter followed up by Cocks [5] who exploited these recoils

as a secondary source of keV-energy Ions for the f i r s t in what is now a

very long chain of ac t ive experiments on such topics as s ing le and

mult iple electron capture by alow,highly charged ions [ 3 . 6 ] -

Meanwhlle, electron cyclotron resonance (ECR ) Ion sources have come on

line, and a new generation of cryogenic electron beam Ion sources (such

as CRY.SIS at Atomfysik) are about to. These latter sources have many

advantages for carrying out such collision experiments, especially In

their intensity v3. charge state characteristics, but lack the attrac-

tively low potential emlttance of the "scalpel" source.

In applications where production of high-charge-state Ions of low

emlttance Is an Important consideration, and low production rates are

sufficient or even desirable - as they often are In trapping applica-

tions - - both the '"hammer" and "scalpel" methods are likely to continue

to be exploited to good advantage. Significant potential benefits of

the "scalpel" technique Include development of low emlttance, sub-ns

pulsed ion sources well suited to coincidence experiments In such areas

as:

o angle-resolved high-charge-state ion-atom and ion-molecule collisions

in the - 10 to few 100 eV range;

© new opportunities to study the interaction of "tunable" radiation

simulating stellar radiation of virtually all temperatures with cold

atoms and molecules (found for example in interstellar clouds);

© techniques Tor making precision spectroscopy on few-elect"on ions

(e.g. Intervals between high Rydberg s tates . Lamb shifts , t . c ) , which

take advantage of the enhanced capabilities for trapping and further

cooling that production of cold Ions makes possible;

o new opportunities to study photolonizatlon of stored or confined

Ions;

o development of new methods to study molecular Interactions with

surfaces, and molecular stereochemistry;

©. study of site- and element-specific radiation damage at highly

localized sites.

Further valuable development of the "hammer" method may also be

forthcoming in the forseeable f u t u r e . The p o s s i b i l i t y of producing a

s i g n i f i c a n t l y more in tense, pa ras i t i c source of bare and few-electron

low-energy Ions (>1O IZ extracted Ions /sec) , "hammered" at MHz frequen-

c ies In a heavy-Ion storage r i n g , has recent ly been conjectured by

U l l r i c h et a l . [ 7 ] .

Despite the abundance of activity in the field of recoil Ion

production and use and the Importance of the parasitic-Ion-source

possibi l i ty, few data concerning the energies and angular distributions

of the primary recoil Ions have been published [8 ] . Ullr ich et al [8]

have recently published a measurement of the entire recoil-energy

distribution Tor NeJ* Ions produced by 1.1 Mev/u U projectiles, using a

specially designed time-of-flight (TOF) spectrometer. Substantial new

knowledge of intr insic Interest, as well as practical significance for

design of parasitic sources, can be gained from measurements of recoil

energies and angular distributions.

In t h i s paper we review recent measurements of mean Ion recoil

energies Er carried out using the scalpel method at the Stanford

Synchrotron Radiation Laboratory, and using the hammer method at the

OP.NL EN tandem accelerator f a c i l i t y . As most of the data and detai ls

concerning the methods have been covered In recently published or soon

to be published papers [8,9,10,11] we concentrate here on summarizing

the results and Interpreting them In the context of the principal

concern of th i s meeting, the physics of stored and trapped p a r t i c l e s .

Further technical d e t a i l s may be found i n the references [ 8 ,9 .10 ,11 ] .

2. Oetemlnation of mean recoil Ion energies E r .

2 . 1 . "Scalpel" method determinat ions.

The "sca lpe l " technique Involves use of a t l m e - o f - f l l g h t (TOF)

analyzer s imi la r in de3ign to that used by our colleagues L. L l l j eby

and H. Danared at Atomfyslk In Stockholm, to permit j o i n t examination

of ion photoproduetton rates and energies •• hrough study of the yields

and widths of the corresponding TOF peaks. Thea*; peaks arise froc: the

arr ival of ions at a dual channelplate detector S 1 us subsequent to

their creation by a synchrotron radiation x ray burst In a dilute gas

sample (-0.2 mT) maintained In the extraction region at the front end

of the analyzer. The short length of the analyzer (- 10 cm) Is

Important to keep n i g h t times of Interest In the range between

successive x-ray bursts. Also, event s tat is t ics benefit from a short

timing cycle, since more cycles are accommodated In the beam time

available.

An example of a TOF spectrum for Xe, irradiated by white x rays

from SSRL beam l ine IV-2 are shown in Fig. 1. The timing resolution

achieved sufficed not only to make the desired temperature measurements

but also to achieve adjacent mass Isotoplc resolution of the charge

3tate (r) spectra of Xe. This fact alone qualitatively Indicates low

Ion temperatures — otherwise the TOF spectra would be hopelessly

smeared.

In our I n i t i a l study [9 ] , beams of white (and later monochromatic)

x rays from an 8-pole wiggler operated at 15 kG were focuased by a

toroidal mirror and colllmated to a 1-mm diameter x-ray beam at the

position of the gas target. The c r i t i c a l energy of the radiation was 1

keV (corresponding to the 2-GcV electron-beam energy) and was

attenuated below 3 keV by Be windows. This target was viewed by the

vert ical ly mounted TOF analyzer through a jj~mm long s l i t located Just

above the x-ray beam. X-ray intensity was monitored by ion chambers

positioned both up and downstream of the uhv system housing the

analyzer. Typical f lux through the target was - 10'2 photons mm"1

sec"', and total ion counting rates In the TOF detector were - 1-5

kHz. Detector backgrounds from stray x-rays were found to be

n e g l i g i b l e (< 3 Hz). The x-ray throughput was s u f f i c i e n t to permit the

use or a S i ( l l i ) double c rys ta l monochromator (bandpass 1 x 10~s) to

tune r a d i a t i o n above, below, and i n te r l eav ing the L, , L j , L, edges of

Xe to explore e f fec ts of tuning the x rays through an absorpt ion edge.

Measurements of mean charge s ta te r are presented i n Table I . The

mean charges l i s ted can be compared with pioneerln/j mass-spectroscoplc

measurements carried out long ago using x-ray guns and f i l t e r s by

Carlson et a l . [12J. and ( for the case of Xe) very recent measurements

by Tonuma et a l . [13] using monochromatic x rays from the Photon

Factory. In general, the agreement between our resul ts and the

measurements of Tonuma et a l . Is 3een to be excellent, resolving a

discrepancy we observed [9 ] In the mean charge state increment measured

for Xe as the L, edge Is crossed. Consideration of Costei—Kronlg

yields [1H] for transfer of an L, vacancy to the L 2 | , subshells and of

r e l a t i v e p h o t o i o n i z a t i o n cross sections of the L 1 ( l i , l e v e l s [ 1 5 ] leads

to an est imated s h i f t i n mean charge s t a t e on the order of tha t seen by

our group and by Tonuma et a l .

Mean recoil energies were determined through study of the widths

of the corresponding TOF peaks, using procedures similar to those

described earlier [ 9 ] . For each charge state r the mean i n i t i a l energy

Er was deduced from a f i t of the peak widths at FWHM to the form

At2 - a • B/(r<) • Y / ( r<) 2 . (1)

The constants a. B, Y represent contr ibutions to measured f l i g h t time

from, respect ively, t iming uncertainty; f l i g h t time var iat ions due to

f i e l d f r i ng ing ; and reco i l - i on energy [9 ,16 ] . The e l e c t r i c - f i e l d

scaling parameter (<) represents six d i f fe ren t choices of

proportionately scaled L9,16 ] e lect r ic f i e l d values in the TOF

analyzer. The e33enti.1l correctness of th<1 Ansatz for At." wag well

ver i f ied by the clustering of TOF data around f i t t ed quadratic curves,

as was i l lus t ra ted In Fig. 2 of [9] for the rare gases ionized by

synchrotron radiat ion, ind in Fig. 2 below for Ar* r recoi l ions

produced by 23 MeV Cl*4 incident pro ject i le Ions and detected in

coincidence with emergent Cl+ < projecti les (simultaneous t r i p le

electron loss from the pro ject i le) . Comparative recoi l energies

determined in this manner are summarized in Table I I , comment on which

is deferred unt i l these "hammer" measurements are summarized.

2.2 "Hammer" method determinations

A nearly identical TOF technique has been used to measure the energies

of recoi l ions produced using the "hammer" technique. Beams of - .5 -

1 Mev/u C l + J produced by the ORNL. EN Tandem have been used to create

argon and neon recoi l Ions which have been observed both in "singles"

mode (averaged over a l l scattered Ion charge states), and In coinci-

dence with Individual scattered Ion charge 3tates ranging from 6 to 8.

The nearly symmetric Ion-atom col l is ions discussed here correspond

to impact parameters - .5 au. [8,10]. Recoil energies for similar

Impact parameters estimated by simple e last ic Coulomb scattering

theory, in which EP « (q r ) 1 , where q and r are the pro ject i le and

recoi l charge states, respectively, are an order of magnitude smaller

than those we observe. When present measurements of log recoi l energy

are plotted vs log qr (q - 5, r - 6,7,8,9), a surprisingly linear

dependence Is observed, suggesting a power law of the form Er « (qr)n

where least-squares f i t results give n - 10. This extraordinari ly

steep dependence 13 due, in part, to the low q (q - 5) used in the

f i t s . Since project i le loss cross sections are large compared to

capture cross sections Tor C l + i at these energies, most argon recoils

are accompanied by scattered project 11 os of charge state higher than

f ive. Similar plot.:; for the data obtained in coincidence with scatter-

ed projecti le charge state suggest an exponent n - 5, as i l lustrated In

Fig. 3. This s t i l l strong dependence, far steeper than the n - 2

obtained for elastic Coulomb scattering is due primarily to the fact

that effective charges seen by the target and projecti le nuclei at the

moment of closest approach are or much greater importance In determin-

ing recoi l energy than are the asymptotic values (q . r ) . In fact, good

agreement with present measurements Is obtained by using effective

target and project ILe charges obtained by Hartree-Fock calculations

[8,10] to allow for the actual screening prevailing near the moment of

closest approac.i.

These results i l lust rate a hazard in using a familiar and often

useful rule-of-thumb estimate provided by Olson, Schlacter et a l . [17]

for estimating recoil energies:

Er ( in eV) = i4x10--qzrI/(MTEpb1), (2)

with My the target mass in u, Ep the projectile energy in MeV/u, and b

In units of as> and q and r the asymptotic final charges. This

estimate coincides with the formula which is obtained for simple

elastic Coulomb scattering for an approximately "half-Coulomb" col l i -

sion (the target being neutral on the incoming ha l f ) . I t originally

arose, however, from Olson's parameterization [17] of some classical

trajectory Monte Carlo (CTHC) calculations he had made in calculating

Inelastic multiple lonlzatlon cross sections for 1 MeV/u collisions of

bare Ions (q - 2-20,4*0 with the rare gases, assuming a projecti le

velocity appreciably greater than that of the target electrons (not the

case here).

For the data discussed here, this estimate und'Testimates recoil

ion energies by nearly an order of magnitude -it an Impact parameter of

the L shell radius of either collision partner. What Is different Is

the extensLve shell interpenetratton accompanying the collisions

discussed here, In contrast to large impact parameter lonlzation by

bare nuclei passing well outside target shell rad i i , the case of

principal interest In Ref. [17]. One anticipates that Eq. (2) provides

a plausible and useful estimate for very highly charged, fast heavy

ions achieving complete or nearly complete target electron stripping of

an atom of signif icantly smaller nucle;ir charge. But for a more nearly

symmetric col l is ion, or in a situation where removing nearly a l l target

electrons eff ic ient ly is a paramount consideration — as is l ikely to

be the case in trapping experiments on heavy 1- , 2-. or 3~electron ions

— then Eq. (2) may underestimate the recoil energy by an order of

magnitude or more. The "hammer" data In Table I I and Fig. 3 indicate

this trend. Ions produced using the "scalpel" method are seen to be

s t i l l colder from this relative standpoint.

3- Further remarks on the "scalpel" method; relevance to future

trapping experiments.

While the "scalpel" method does Indeed provide substantially

colder ions than the "hammer" method, the charge state reached is

limited by the element-specific lonlzation state reached when the

Auger emission cascade, and associated electron shake-up and shake-off

phenomena, have subsided. At currently available synchrotron radiation

intensity levels, the time needed to achieve further lonlzatlon by

sequential ionlzation of outer shells, or a second photolonization In

an inner shell in the same atom, would seem to require trapping.

Estimates of lonlzatlon states reached and the times required '..u

achieve them have been discussed In earlier papers by Jones et a l . [18]

who considered the case of sequential phoLol oni zat. ion only; and by

Church et al. [19], who considered i hybrid scheme based on a combina-

tion of Inner-shell photoioni. zation plua sequential stripping from

outer shells.

That our Initial efforts tor achieve trapping of multiply charged

argon Ions In charge states 2* to 6- by photolonlzatlon of argon atoms

In a Penning trap have been successful Is illustrated In Fig. «. Since

these data were discussed earlier In this conference by Prof. Church,

there 13 no need for further elaboration of them here.

Instead, we conclude by comparing and contrasting various aspects

of the hammer, scalpel, and sequential lonizatlon techniques as they

pertain to present and potential future measurements on high charge

state Ions stored In Penning traps. The latter two methods are

superior to the former Insofar as they produce Ions close to room

temperature, which In turn permits trapping at low well depths, a

circumstance which obviously eases the task of potential further

cooling. Since the hammer excitation method Involves pumping with

beams of high charge state Ions, and these are awkward to transport

across the axial magnetic field lines characteristic oi a Penning trap

configuration, the hammer method involves creation of Ions over the

entire axial gap between the trap end caps, following passage through a

magnet yoke and pole tips as well as the end caps themselves. Pumping

with x-rays instead permits radial injection, which Is not only easier

to achieve geometrically, but also permits creation of ions over a

sub-mm range of axial distances, limiting the axial energy spread of

the subsequent stored Ion motion.

A hybrid photoionlzation scheme, where the Initial step consists

of a vacancy cascade initiated by an Inner shell photolonlzatlon event,

permits production of an Initially fairly high charge stage, e. g. r

typically In the range 5 - IS. One thus avoids rilling the trap with

apace charge I'rom low charge states, a ri.sk run by both the hammor and

sequential Ionizatlon methods. Since high r'3 are easily confined by

low magnetic f ie lds, values of B - 0.05 T may be used. Use of iron

free coi ls is thu3 a l ikely possibi l i ty, representing very considerable

savings in cost and vacuum system complexity. The combination of low-

well-depth trapping and small confining f ields allows one to consider a

confinement volume of a few cubic mm, and a typical cyclotron radius

for the Ion motion of p - 1mm per charge.

While low confining well depth obviously eases the problem of

further cooling, for x-rays the product FIT, where Y la the natural

width of a suitable excited level, Is generally far too large to

permit direct appropriation of the laser cooling concepts familiar from

cooling studies at much softer laser photon energies, as discussed

several times earlier In this meeting. At least two alternative

choices exist . F i rs t , "sympathetic" cooling may be tr ied by simultane-

ously confining and laser cooling low-charge Ion3 of similar charge-

to-mass, relying on col l ls lonal energy transfer to cool the high

charge state species of primary Interest. A second possibi l i ty has

been suggested recently by Church et a l . [20] , who point out that

although moat optical transitions in highly charged Ions occur in the

x-ray range, selected transitions occasionally f a l l in the range of cw

or pulsed dye laser wavelengths. For example, several transitions

between f ine structure levels of the ground states of multiply charged

argon lens occur at wavelengths - 500 nm. Since for room temperature

Ions Doppler widths correspond to < 1 GHz, laser cooling on such

transitions may be possible (though at relat ively slow rates).

Acknowledgments

This work was supported In part by the U. S. National Science

Foundation; by the F. R. G. Deutsche Forschungsgemelnschaft; and by the

U.S. Department of Energy under contract No. DE-AC05-81OR21U00 with

Martin Mariet ta Energy Systems, Inc. and under contract Ho. DE-AC02-76-

CH00016. I t was performed a t SSRL which Is supported by the

U.S. Dept. of Energy. Office of Baste Energy Sciences, and the National

I n s t i t u t e s of Health, Biotechnology Resource Program, Division of

Research Resources, and a t the ORNL EN Tandem F a c i l i t y .

•Permanent address: Unlversl ta t Frankfurt, Frankfui i , F.R.C.

12

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15

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Huld t , S . , Johansson, S . - E . , N i l s son , E . , and Church, D. A.,

Phys . Rev. L e t t e r s 56, 2614 (1986) .

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S. B . , Gibbons, J . P . . S e l l i n , I . A., and Schmidt-Bocking, H., In

Ref. 6 , p . 432.

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I . A., L H J e b y , L . , Hu ld t , S . , Johansson , S.-E., N i l a son , E. and

Church, D. A., in P roceed ings of a U. S. - Japan J o i n t Seminar on

t h e I n t e r a c t i o n s of Highly Ionized Atoms Produced by Ion- Atom

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in Nuc l . I n s t . and Meth. B.

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(1981) .

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14

18. Jones, K. W., Johnson, B. M., and Meron, M., Phya. L e t t . 97A, 377

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I . A . , J . Phys. B17, 1^01 ( 1 9 8 t ) .

20 . C h u r c h , D- A.., K.ravLs, S, 0 . , He rou , H . , 4otu\aov\, 6 . H . , Jones,

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(1988 ) .

15

TABLE I . Average charge state r following inner-shell photolonizatlon.

Energy of monochromat.i zed x rays (In eV) is indicated for each mean

charge.

DeepestVacancy

Ne K

Ar K

Kr L,

Xe L,

Xe L,

Xe L,

Xe M

Present

2.3 (white)

4.2 (white)

5.4 (white)

7.8 (5475)

7.7 (5400)

7.5 (5050)

6.2 (4750)

r (expt.)ref. 12

2.3

4.2

6.7

9.1 (8400)

7.4 («960)

6.5 (1500)

7

7

7

6

ref. 13

.74 (5800)

.65 (5450)

.45 (5100)

.01 (4500)

16

Table I I . Comparison oi' argon recoi l Ion energies (In eV) produced

using unmonochromatlzed x rays from SSRL, with those produced using 23

and 27.6 MeV Cl*1 beams from the ORNL EN tandem accelera tor detected in

coincidence with Cl*cl where q-7.8.

Argoncharge

2

3

1

5

6

7

8

9

SSRLx-rays

O.OH6(2O) eV

0.050(23)

0.066(21)

0.069(35)

0.086(36)

0.092(71 )

0.081(56)

Cl

.27(

. « (

.8H(

2.08(

21 MeV

.15)

.16)

.21)

.50)

Cl

•

•

1 .

2 .

H.

Cl

* l (

63(

11(

26C

20(

+ •

.12)

.12)

.25)

• 35)

.76)

27.6 MeVCl* 7

<.2O

<.H0

.H7(.28) 1

1.66(.51) 2

C l * '

.21 (

-35(

.71 (

• »3(

.71(

.09)

.12)

.21)

• 3«D

.78)

17

Figure Captions

Fig. 1. Xe charge s ta te distribution following L-shell vacancy produc-

tion using synchrotron radiation. Plotted Is number of Ions detected

as a function of flight time. Low Ion temperature Is Indicated by the

nearly complete Isotoplc resolution observed.

Fig. 2(a). Spectrum of Ar*r recoil Ions produced by 23 MeV Cl*5 and

detected In coincidence with Cl*8. Inset shows FWHM of Ar*8. Ar*9.

F'.g. 2(b). Quadratic behavior of the square of Ar*r TOF peak widths

plotted as a function of (r*:)"1 (see t e x t ) .

Fig. 3<a). Dependence of Er on product (qr) of f trial projectile and

recoil charge states for 1-, 2-, and 3-electron loss from 23 MeV Cl*5

projectiles Ions.

Fig. 3(b). Er vs qr for beams of 23. 27.6 and 33 MeV Cl*5. Data have

been r i t ted to straight lines (see t ex t ) , both shifted Tor

c la r i ty . Both scales are logarithmic.

Fig. 4. Argon Ion q/ra signals obtained from ions stored in a 5 V deep

Penning trap pumped by white x-rays from the Brookhaven National Light

Source.

18

129

X(\l^-»—re)--—»-

A3

X

— —'— • 1 , —

_ _ — — -

" T.

T

03X

. • — " —

" " - ~ ^

— -

X ^^~

•2 <V

X

-

--)

x %

o

co

X

O

504

o

oLU

I

SXNRO0

FLIGHT TIME (ns)700 500

oo

CO

oo

<MCO

c

X

300

70

siNnoo