Synchrotron radiation in the infrared

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Synchrotron radiation in the infrared

P. Meyer, P. Lagarde

To cite this version:

P. Meyer, P. Lagarde. Synchrotron radiation in the infrared. Journal de Physique, 1976, 37 (12),

pp.1387-1390. �10.1051/jphys:0197600370120138700�. �jpa-00208539�

SYNCHROTRON RADIATION IN THE INFRARED

P. MEYER and P. LAGARDE

Laboratoire de

Physique

des Solides and L.U.R.E.

(*)

Université Paris

Sud,

⁹¹⁴⁰⁵

Orsay,

^France

(Reçu

^{le 1 er}

juin 1976, accepté

^{le 3 août}

1976)

Résumé. ²⁰¹⁴ Nous avons

entrepris

^{des mesures}

photométriques

^{dans la}

partie infrarouge

^du spectre rayonné par ACO, l’anneau ^de

stockage

d’Orsay, pour vérifier la théorie dans ^ce domaine.

Par rapport ^{aux sources}

classiques

^{utilisées en} spectroscopie infrarouge ^une

augmentation

^{de l’in-}

tensité de plus d’un ordre de grandeur peut ^être espéré ^{en utilisant une} ligne

spécialement

^conçue.

Abstract. ²⁰¹⁴

Experimental photometric

measurements have been

performed

in the infrared part of the spectrum of the radiation emitted

by

ACO, ^the storage ring ^of Orsay, ^{in view to check the} theory ^{in this} range. An improvement ^{of more than one order of}

magnitude

ⁱⁿ intensity

compared

to classical sources used in infrared spectrometers ^{can be} expected ^{with a}

specially

designed ^line.

Classification

Physics ^Abstracts

0.642 ^- 0.690 ^- 2.240

1. Introduction.

- Storage rings

^and

synchrotrons

are now well-known sources for the ultraviolet and

X-ray

parts ^{of the}

photon spectrum.

^The

theory

^of

the

synchrotron

^radiation

production

^works very well in these _ranges and we may ^now ask : is the

synchro-

tron radiation as

powerful

^at the other end of the

spectrum compared

to the best available sources as

it is in the ultraviolet ^or the

X-ray

^{domains ?}

A first attempt has been made

by

Stevenson et al.

[1]

but

they

^use in their calculations some

geometrical

factors which do not seem realistic for infrared _spec-

troscopy,

and there is still a lack of

experimental comparison

^{between a} classical infrared source and

a storage

ring.

In this note we

present

the results of

experiments

done at

ACO,

^the storage

ring

of the linear accelerator of

Orsay,

otherwise used as an ultraviolet _source,

compared

^{to a} standard Globar source and the theore- tical

expression

^{for the}

synchrotron

^radiation gene- ration in the

long wavelength

^limit.

2. Theoretical

expressions

^{for the}

emission-spectra

of both sources. ^-

Following Schwinger [2]

^{or Sokolov}

and Ternov

[3]

^the

intensity

^{of the}

light

^emitted

by

an electron of _energy

E,

^{with rest} energy

mo c2,

^on

a circular orbit of radius R can be

expressed by, [4]

where P is the _power radiated into the part ^of space defined

by

^{a vertical}

angle V/

^on both sides of the

orbit,

^{and an}

angle

(p

along

this orbit.

e the electron

charge

ⁱⁿ

Coulombs,

so the dielectric constant,

R the

magnetic radius,

y ⁼

E/mo C2,

A the

wavelength

^of

interest,

AA the

spectral

^width

considered, Âc

the critical

wavelength

KS/3(x)

is the modified Bessel function of order

5/3,

Integrating over qf

^{between 0 and}

n/2,

^{we obtain the}

total _power emitted

by

^one

^electron,

into all the vertical

plane

per mrd of orbit.

If there are n electrons

circulating

^{in the} storage

ring, equivalent

^{to a current}

I,

^this power is

given by

or,

by chosing

suitable units for

R, I,

^{E and}

(*) ^{Service commun} C.N.R.S. Universite Paris-Sud.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0197600370120138700

1388

In the limit of

long wavelengths

the entire

expression

in

straight

^{brackets may be}

replaced by

^an

asymptotic

one we shall

give

later. The number of

photons

emitted _may be derived

by using

the relation

Conventional infrared ^{sources are}

commonly

^com-

pared

^{to black}

body

emission. The

following

expres- sion

gives

^the

intensity

of radiation at the

wavelength

emitted

by

^a surface of area s of a black

body

^{at the}

temperature ^{jT into a solid}

angle

^{of 1} steradian around the normal of the surface.

or

again

with suitable units for

A5l,

5l and s :

Eq. (1)

^and

(3)

^{are all we need for a}

comparison

^of

both _sources, but the

problem

^{can be}

simplified by assuming

^{that we use the same} resolution r in both

cases. We may then write AA = rA and the

quantity

we are interested in is

Synchrotron

^radiation

A _

( 1

^{mrad of}

^orbit,

all vertical

plane)

.

"

Black

body (1 cm2,1 sr)

°

Or, by replacing K5,3 ^by

^its

asymptotic expression

for _x

1,

^i.e.

A,IA

¹

and

integrating,

^we may write for the term in

straight

brackets in

(2)

and obtain

finally

This

expression

^{can be} used if the theoretical

expression

^for

synchrotron

radiation is

experimen- tally

verified in the infrared.

3.

Experiments

^and

comparison

^{with the}

theory.

^- The

experiments

have been done ^at

ACO,

^whose

bending

radius is R ⁼ 1.1 m. The stored current

was 70 mA at 0.54 GeV. The

experimental

setup ^is schematized in

figure

^{1. The infrared}

monochromator,

a

Coderg

^SV type, ^{uses a}

grating

^{of 300}

grooves/mm

and a silicon or

germanium

transmission filter. The detector is a

Golay

cell made

by Eppley

^{with a dia-}

mond window. In front of the entrance

slit,

^{we used}

a

chopper

^whose

frequency

^{was near 10 Hz. The}

whole

apparatus

^{is in a vacuum} better than

10-4

torr.

FIG. 1. ^- Experimental setup ^for intensity measurements on

ACO Mo : splitting mirror, M1 : focusing mirror, C ⁼ rotating chopper finger, M. = grating monochromator, ^{D = detector mo-}

dule with f = filter and d ⁼ Golay-cell ^detector.

Because of

experimental

limitations on the vacuum

line the beam

coming

^{from ACO is}

only

^{3 mrad hori-}

zontal and

± 1.5

mrad vertical wide.

Therefore,

even if the

geometrical

units used in the ratio A

(1 mrad,

¹ sr, 1

cm’)

^are

nearly

^{usual values for}

infrared spectrometers ^and

synchrotron

^radiation

pipes,

^{we choose a}

layout

^{for the}

experiments

^done

with the

globar

which is unrealistic but _very close to the

previous

^{one : the source is}

placed

^{1.5 m before}

the mirror

M,

^{and we use two}

pin

holes in order to obtain a beam

geometry comparable

^{to the first case;}

the solid

angle

^of aperture is then limited to 1.07 ^x

10-4

sr and the useful surface to 3.59 mm’.

Both

experiments

have been done in the _range

1.5-3 Jl

^{since we need}

only

^one

point

^{for an absolute}

comparison ^and,

because of the solid

angle used,

^we

must work in this domain where the _power emitted

by

^the

globar

is maximum. We used either a germa- nium or a silicon filter but in this

frequency

range, the

germanium

^cuts all the second order and is more

suitable;

^{in all cases} the slits of the monochromator

were set to 3 _mm, which

gives

^a theoretical resolution of about 100.

Assuming

^{that a}

globar

^{is a black}

body

^{with an}

emissivity

^of

85 %

^{at this}

wavelength [5]

^{and a mea-}

sured temperature ^{T = 1 300}

K,

^we obtain the results summarized in table I :

I.

Signal (in volts)

measured with the

globar

^{as a}

source.

II.

Signal (in volts)

measured with ACO

(540 MeV,

70

mA).

III.

Expected

theoretical power emitted

by

^a

globar

in the same conditions from _eq.

(3’) using

^the

right

values of the solid

angle

^{and the}

emissivity.

IV. Ratio

I/III :

response of the whole spectro-

meter.

V. Ratio

II/IV : experimental

^{measure of the}

power emitted

by

^ACO.

VI.

Expected

theoretical _power emitted

by

^ACO

from _eq.

(2)

used with the suitable variation versus

the vertical

angle.

Instead of

calculating

^{the ratio}

A, comparison

between columns V and VI

gives

^the

check we are

looking for,

and will be discussed in the next

paragraph.

4. Discussion. ^- From table

I,

^{we can see that} the agreement ^between

expected

^and

experimental

results is

satisfactory,

^{if we}

keep

in mind the

following

features :

- The _power

entering

the monochromator is _very low and the poor

signal

^to noise ratio makes a

precise knowledge

^{of the}

signals

^difficult.

- The

experiments

^on ACO have been done with

a first

splitting

^{mirror at the entrance} of the line whose

reflectivity,

because of

previous experiments

^{on this}

line,

^is

definitely

^{not 100}

%

^{at all}

wavelengths

^{and the}

aperture

may be

slightly

different from the values we

used.

Nevertheless,

the fit is

good enough

^{to allow us to}

assume that the

theory

^of

synchrotron radiation,

which has been checked in the UV-soft

X-ray part

of the

spectrum

^can be used in the infrared _range.

Thus, figure

²

gives

^a

comparison

^{between two com-}

mon infrared sources and a

planned experiment

on ACO.

1)

^{Globar source at 1 300}

K,

^0.30

cm2

of surface in

conjunction

^{with a} monochromator of

aperture f/2.3 [5].

2) High

pressure mercury

lamp (HPK 125)

^used

with the same

aperture [6, 7].

’

3)

^{ACO with a modified vacuum} chamber which allows us to

intercept

^an horizontal

angle

^{of 300 and}

collect almost all the useful vertical

angle.

^{We can see}

the

improvement

^obtained

by using

^{ACO as an infrar-}

ed source versus conventional ones : at least two orders of

magnitude

^{in the}

long wavelength

range, with the

possibility

^of

using

^a convergent ^{mirror in} order to obtain a very

bright

^{source. This is}

^due,

^to

FIG. 2. - Emitted intensity ^{of two classical sources} compared

with the synchrotron ^radiation (1 J.I. ^band pass). a) ^{Globar source}

0.3 cm’ at 1 300 K and an aperture ^of f/2.3 ; b) ^HPK-125 high

pressure mercury ^arc with the same geometrical conditions; c) ^ACO with 30° of orbital angle and all the vertical plane running ^{at 100} mA,

0.54 GeV.

the fact that ACO is a small

ring,

^{with a short}

bending

radius

( 1.1 m compared

^{to 12.7 m at}

SPEAR),

^which

permits

^a

large

part of the orbit to be

intercepted :

no

comparable setup

^can be obtained near

larger storage rings

^{as DCI or} SPEAR. A future

improve-

ment

planned

^{at ACO is an} increase of the stored current to the _range 200-400

mA,

which should

give

another factor of 5 in favor of the storage

ring.

5. Conclusion. ^-

Experimental

checks have been done

carefully

^{in order to} compare ^a classical infrared

source

(a globar)

^with

synchrotron radiation; they

allow us to say that the

Schwinger theory

^{of the}

synchrotron

radiation works in the infrared _range.

Then, commonly

^{used sources} appear ^to be 1 or

2 orders of

magnitude

^less

powerful

^{than a}

specially designed port

^{at the}

Orsay

storage

ring ACO;

^even

if

larger

^{and more}

powerful

storage

rings

^{such as}

DCI or SPEAR are as

bright

^as

ACO,

the latter appears ^to be more useful because of the

large

^solid

angle

^{which can be}

intercepted.

But,

in view of

spectroscopic

work with this source we must answer some other

questions concerning

1390

stability

^{and noise}

figure.

^From

figure 2,

the emitted power in ^a

1 J.1

band pass is still very low for conven-

tional

detectors;

^{then the use of a Michelson} type interferometer cannot be avoided in ^a first step. ^It

seems hard to think also

that,

^{in the near}

future,

^fart

infrared detectors will be fast

enough

^to be monitored with the

pulsed

^{structure of the}

synchrotron light (1 pulse

every 77 ^n.s. in the case of

ACO).

^Neverthe-

less,

^{the main}

advantage

^of

synchrotron

radiation in

the whole _range of

wavelength

^{is its noise} characteris- tics : all measurements show that the

only

^noise

limitation is the

photon counting statistics,

^which

shows

along,

with the emitted _power, the usefulness of this source in

spectroscopic

^work.

Finally,

^{we must}

point

^{out that the} lifetime of the electron beam in ACO is

typically

¹⁰

hours, making

the correction of line base deviation in Michelson

interferometry quite

easy.

References

[1] STEVENSON, ^J. R., ELLIS, ^{H. and} BARTLETT, R., Appl. Opt. ¹² (1973) 2884.

[2] SCHWINGER, J., Phys. ^{Rev. 75} (1949) ^1912.

[3] SOKOLOV, A. A. and TERNOV, I. M., Akademie Verlag, ^Berlin (1966).

[4] WUILLEUMIER, F., Le rayonnement synchrotron émis par les

anneaux de stockage d’Orsay Rapport ^LURE 74/03, Orsay, 1974.

[5] PLYLER, ^E. K., YATES, ^{D. J. C. and} GEBBIE, ^H. A., ^J. Opt. ^Soc.

Am. 52 (1962) ^859.

[6] KIMMIT, M. F., Roy. Radar. Estab. Techn. Note n° 716 (1965).

[7] BOHDANSKY, J., Z. Phys. ¹⁴⁹ (1957) ^383.