BA.R.C.-1440

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•cic^Ha *Ji*«n

DESIGN AND FABRICATION OF A POLYCHROMATORUSING PASCHI-N-RUNGK MOUNT

R. I' Shukla. P. S Murtv. S. S. Biswas, P. K. S. P. Rao,A V Sankaran anil S. L. N. (1 Krishnamaohari

Speciroscopv Division

1988

B.A.R.C. - 1440

GOVERNMENT OF INDIAATOMIC ENERGY COMMISSION

o

Uof<CO

DESIGN AND FABRICATION OF A POLYCHROMATOR

USING PASCHEN-RUNGE MOUNT

by

R.P. Shukla, P.S. Murty, S.S. Biswas, P.K.S.P. Rao,A.V. Sankaran and S.L.N.G. Krishnamachari

Spectroscopy Division

BHABHA ATOMIC RESEARCH CENTREBOMBAY, INDIA

1988

B.A.R.C. - 1440

INIS Subject Category : E4200; B1120

Descriptors

SPECTROMETERS

FABRICATION

TRACE AMOUNTS

CADMIUM

NICKEL

COBALT

BORON

MANGANESE

CHROMIUM

MAGNESIUM

URANIUM

IMPURITIES

PLASMA

DIFFRACTION GRATINGS

WAVELENGTHS

EMISSION SPECTROSCOPY

PHOTOMULTIPLIERS

OPTICAL DISPERSION

SPECIFICATIONS

FOCUSING

MULTIELEMENT ANALYSIS

QUANTITATIVE CHEMICAL ANALYSIS

ABSTRACT

The design and fabrication of a polychromatot for the wavelength range

2000 A to 4000 A is described. It consists of a holographic concave

grating of radius o£ curvature 1 metre and frequency 2160 grooves/mm.

The Pti&chen-Rumjfc type of mount, was chosen because it. har. a widur

spectral range. Holographic diffraction grating was chosen to reduce

(he. amount of stray radiations and eliminate completely the ghosts.

The linear reciprocal dispersion on the Rowland focal surface is about

4.G A/mm. The resolution of the polychromator is about 0.25 A. Seven

slits are fixed precisely on the Rowland circle to the corresponding

wavelength positions of certain sensitive lines of the elements Cd,

Ni, Co, B, Mn, Mg and Cr. The radiation of each wavelength is detected

by the photomultiplier tubes. The polychromator will be used with

inductively coupled plasma (TCP) as a source of excitation for the

detection of impurities Cd, Ni, Co, D, Mn, Mg and Cr in different type

of samples.

DESIGN AND FABRICATION OF A POLYCHROHATOR USING

PASCHEN-RUNGE MOUNT

by

R . P . S h u l t l a , P . S . M u r h y , S . S . n i i s w d : ; , P . K . S . P . R a o ,

A . V . S a n k a r a n a n d S . L . N . G . K r i s h n a r a a c h a r i

1. INTRODUCTION

Opt.ical emission spectroraetry (OKS) is a widely employed

technique for the analysis oi various trace elements. Electrically

generated 'flames' (plasma) have proved Lhat. simultaneous uulUelement,

analysis can be successfully carried out with better detection limits

and precision t.han before . Thic i& nainly due l.o Lhe jnl.zoducl.inn of

2-9inductively coupled plasma (ICP)' as the excitation source for the

emission spectrometry. In recent years, TCP-OES has become a sought-

after technique for the analysis of solution samples bceaur.c- of. it.s

many features such as high analytical precision and accuracy, high

sensitivity, large concentration dynamic ranye, high analytical speed,

capacity to excite all elements simultaneously without any

interference of coexisting eleiients and simple operation. Therefore

the project of indigeneous fabrication of the 'Direct Reading

Spectrometer* to be used with the inductively coupled Aryon plasma as

th<; excitation source was taken up by Spectroocopy Division for the

detection of seven elements namely Cd, Id, Co, D, Mn, Cr and My in

uranium and other types of saaplt>:>.

A polychromator has been designed and fabricated using a concave

holographic grating as dispersing and focussing eleaeni. The present,

teporl: discusses the design and fabrication aspects of the

polychrometor. Several mountings10 were considered and ultimately the

raschen-Runge11 type of mounting was chosen for the following reasons:

Firstly, the spectra are focussed sharply along the Rowland

circle and it. has a wider spectral range.

Secondly, spectral region normal to the grating is the best due

1.0 the property of most uniform dispersion and least amount of

astigmatism. »

Holographic type of diffraction grating was chosen to reduce the

amount of stray radiations, ghost images and to get a high value of

signal to noise ratio. A variable precision entrance slit. war. chosen

to achieve the required amount of resolution. Details of the

polychromator are presented in this report.

2. PASCHEN-RUNGE MOUNTING - PRINCIPLE AND SPECIAL FEATURES

The principle of the concave grating is well known. When a

spectral .source (a slit illuminated by it) is situated on the Rowland

circle of a concave grating, it* spectrum appears in sharp focur. on

that circle itself (see Fig. 1). The Rowland circle is defined as a

circle tangential to the concave grating and of diameter R, where R is

the radius of curvature, of the grating. The absolute values of the

wavelengths ar* given by the following equations:

GRATING EQUATION m> = ( Sin i + Sin e ), (1)

RECIPROCAL LINEAR niRI'ERSION dX/dl = d Cor. 6/ioR, (2)

Where K is wavelength of radiation, s i s diffraction tirdc-r, d is

groove spacing of the <jr«itimj, i is angle of incidence and B is angle

of diffraction.

The signs of angles i and 8 are opposite when they lie on

different sides of the grating normal.

The optical diagram of the polychromator using Paschen-Runge

mount is illustrated in Fig. 2. The entrance slit and exit slits are

fixed on the Rowland circle. The concavs grating is also fixed rigidly

with respect to the entrance and exit slits. We have chosen an angle

of incidence of .11° for a concave gratiny of frequency 2160 grooves/mm

to cover a wide spectral range of 2000 A to 4000 A in the first order.

The spectra are sharply focussed on the Rowland circle of 1.0 m

diameter. Exit slits for the desired spectral lines are fixed on the

Rowland circle at the predeteruined positions.

3. CALCULATIONS OF THE EXIT SLIT POSITIONS FOR VARIOUS VAVELENGTHS

Fig. 3 chows the r.che»<i!.ic diagram of the Rowland circle on which

concave grating (G)r entrance slit (S) and exit slits (Sj, S2,

S,, S ) are located. The distance of the n exit slit 55 from-» n n

th« entrance slit S along the Rowland circle is given by

Arc SSn = (i-8)°Hi/180° (3)

Jf the angle of incidence- i is given, the angle of diffraction 8

is calculated for a specific wavelength k from Eq. (1). Then Arc SS

is calculated fron Z>i. (3) for a known value of R. Following table

gives a set of couputed values of the exit ulit positions for

isolating several wavelengths. The computation was done for R =• 1000

jnu and d * 1/2160 urn. As the grating io ur.ed in first order, • - 1.

TABLE

CALCULATED EXIT SLIT POSITIONS OF THE POLYCHROMATOR

Position

1

2

3

4

5

6

7

R

9

10

11

12

13

14

Wiiveltfng1.li

(Angstrom)

4358.0 (Hg)

4046.0 (Hg)

3125.7 (Hg)

3021.5 (Hg)

2967.3 (Hg)

2BS3.6 (Hg)

2813.2 (Ci)

2795.5 (Mg)

2605.7 (Mn)

2536.5 (Hg)

2497.7 (R)

2432.2 (Co)

2320.0 (Ni)

22«8.0 (Cd)

Angle of

Incidence i

(Tipqieet.)

31

31

31

31

31

31

31

31

31

31

31

31

31

31

Angle of

Diffraction 0

(Degrees)

25.23233887

21.03252931

9.21346252

7.90933509

7.23265903

6.31135335

5.68446358

2.09399625

2.73938791

1.88227149

1.40188834

0.59113826

-0.79747227

-1.19356170

(i-6)

(Ilegj ee.si)

5.76676611

9.96747069

21.78653748

23.09066491

23 7C734O98

24.6R5R6467

25.31553642

25.90600375

28.26066121

29.11772851

29.59811166

30.40886174

31.79747270

31.19356170

AJC SS -n

(i-0)»R/180

(Will)

100.665

173.965

380.247

403.008

414.818

430.850

441.839

' 452.145

493.241

506.200

516.585

530.735

554.971

561.884

<U/d3

(Angs.tioui/mui)

4.118

4.256

4.461

4.578

4.589

4.5S7

4.5S7

4.609

4.619

4.626

4.628

4.629

4.629

4.629

4. DESCRIPTION OF THE VARIOUS ASSEMBLIES

4.1 ENTRANCE SLIT ASSEMBLY

The precision variable slJL, Cat.No. F 1386 of Hilger Anaiyl.ic.al

Limited is attached a I. one end of a hollow brass cylinder. This

cylinder fit.s closely in a hole drilled inl.o a ^ shaped rectangular

aluminium plal.e. The sliL can be rotated in its own (vertical) plane.

The length of the slit is set parallel to the length of the grating

grooves and locked in this position by a screw provided in the •*•

shaped mount. The slit can also be moved longitudinally in relation to

the grating for fine focussing. Once the focussing if. obtained, the

slit assembly is fixed on the metallic base plate by means of screws.

Table 2 gives all the necessary information about the slit. Fig. 4

shows photograph of the entrance slit assembly.

TABLE 2

SPECIFICATIONS OF THE ENTRANCE SLIT

Supplier of the slit : Hilger Analytical limited, Westwood,

Margate, Kent, CT9 4JL, U.K.

Type of the slit : Variable &lit assembly, Cat. Mo. F 1386 MKr.

Slit opening : Symmetrical

Slit, length •. 0 to 18 mm

Slit width : Slit width variable from 0 to 1.0 mm with

two rates of opening and micrometer drim

calibrated as follows:

For 0.0-0.1 mm slit width, 1 division - 1 pm

For 0.1-1.0 nm slit width, 1 division - 5 |im

Overall dimensions : 75 aim diameter

4.2 GRATING ASSEMBLY

The grating is mounted in a circular type of housing which can be

rotated about horizontal axis in a •*- type of metallic rectangular

brass plate. Centre height of the grating is kept 102 naa. The grating

luount is fixed on a levelling table. The grating normal is made

parallel to the horizontal axis by means of levelling screws. The

grating grooves are set parallel to the vertical axis by observing the

spectrum along the Rowland circle. The centre of all the spectral

images nust be. set at the same height from the base plate. This can be

done by rotation of the grating. When the distance between the

entrance slit and grating is adjusted correctly, grating grooves are

made vertical, the normal to the grating is set horizontal and the

angltt of incidence is made 11 , then the grating assembly is fixed

rigidly on the base plate by weans of Ecrews. Table 3 gives all the

necessary information about the grating. Fig. 5 shows the photograph

of the grating assembly.

TABLE 3

PARTICULARS OF THE DIFFRACTION CRATING USED IN THE POLYCHROHATOR

Supplier of the grating : JOBIN-YVON 16-18, rue-du canal-BP11B,

91160 I,ongjumc.au, France

: Holographic concave reflection grating

: 2160 grooves/nun

-. Glass

: 63.5 •, 12.5 ma

: 58 • uui

: 998.8 mm

: 1700 A Lo 4100 A

Type of the grating

Frequency of grooves

Blank material

Blank dimensions

Ruled area

Radius of curvature

Spectral ronye

Blaze wavelength

Angle of incidence

Efficiency

Resolving power

Intensity of ghost

Coating

: 2800 A

: 31

: 46

: 40

: 48

: 80

: 10

: Al

0

\ at

% at

\ at

\ of

-10 .of

+ Mg

2500

2800

3200

the

the

F̂

A

A

A

theoretical

incident 1

4.3 ROHLAND CIRCLE MOUNT

The Rowland circle uount has radius of curvature of 0.5 in, arc

length of 0.56 m and overall width of 50 nun. A slot of length 500 ma

and width 25 mm was cut through the metallic plate of the Rowland

circle mount. The exit slits are mounted along the arc of the circle.

The slits can be translated for isolating the specific wavelengths.

Once the desired wavelength positions are obtained, the exit slits are

fixed rigidly and locked in these positions. The Rowland circle mount

is then fixed on i:he base plate. Fi<j. 6 shows photograph of the

Rowland circle mount.

4.4 EXIT SLITS

Exit elite are made of two blades which are. fixed on a slit of

length 25 mm, width 5 HUB and opening 1 mm. The distance between the

sharp edges is kept 100 pn. The. exit slits are examined under a high

power microscope for parallelism of its e:Jrjes. The slit width is

checked by a comparator which can measure the width to an accuracy of

4.5 BASE PLATE

The base plate is made out of a mild steel. The. top surface of

the plate is shaped plane to mechanical accuracy. Several holes ate

tapped on it for fixing the various assemblies. The base plate along

with the assemblies is covered with a metallic box of thin aluminium

sheet to avoid leakage of liyht. The size of the plate is 1.25 m x

0.75 m x 0.25 m and the cover height is 0.15 m.

4.6 DETECTION SYSTEM

The detection system consists of photomulUplier tube (PMT), D.C.

amplifier and digital voltmeter (DVM) as shown in Fig. 7. The emergent

liyht from the exit slit falls on the phol.ocat.hode surface of the PMT

and electrons are emitted. The D.C. operational amplifier converts the

photoelectric current to voltage which is proportional to the incident

light intensity. This voltage is measured with the help of a digital

voltmeter. The photomultiplier tubes are mounted on L shaped metallic

rectangular plates in such a way that the full area of the

photocathode surface is illuminated by the incident lighl;. For

obtaining the liuiximma signal, the photomultiplier tubes die diDplaced

laterally as well ao longitudinally about the calculated positions and

then fixed rigidly to the base plate. Table 4 gives the details of the

detection system.

TABLE 4

(A) PHOTOHULTIPLIER TUBE (PMT)

Manufacturer of PMT : HAMAMATSU PHOTONICS KK 1126, Ichino-cho,

Hamamatsu city, Japan

Type and nuuber : R-300 and R-427

Wavelength range : 1850 A to 6500 A and peak at 3400 A for R-300

: 1600 A to 3200 A and peak at. 2000 A for R-427

Type and model

(B) AMPLIFIER-. Operational amplifier uoing IC 8007

Input voltage.

Output voltage

IC) H.T. POHER SUPPLY UNIT FOR PHT

: 200 V, 50 H/.

: 400 V to 1000 V d.C.

Model number

Voltage range.

CD) VOLTHETER

: HIT. 2142, 3x1/2 digital voltmeter

: 100 nicrovolt to 1000 volt d.c.

10

5. FOCUSSING OF Tl£ SPECTRUM

The mercury light, is focussed on l.o the entrance sli I. by means

of a quartz lens of focal length 20 en. The distance between the

entrance slit and the light source; is kepi. 40 cm r.o that, the image of

unit magnification falls on the entrance slit. The entrance slit gives

a diverging beam of light and illuminates the concave graliny. The

diffracted light of various wavelengths focuses sharply along *.he

Rowland circle of diameter 1.0 n. The Rowland circle mount i:> placed

along the Rowland circle. The exact position of focus is determined

experimentally by examining the sharpness of the spectra photographed

.it various positions along the Rowland circle. Once the focussing is

achieved, the calculated positions of the. wavelengths of Hg, Cd, Ni,

Co, Be Hn, Hg and Cr are aaiked on a thin phosphor-bronze strip placed

on the focal plane (Rowland circle). Vertical slits corresponding to

these lines are cut at the narked positions of this strip. With the

help of Cd and Hg discharge lamps, the theoretically calculated

positions of Hg spectral lines are verified. Similar checks on the

'correctness of the other slit positions arc carried out using hollow

cathode lamps of the other elements. Fig. 8 shows photograph of the

polychromator.

11

6. SPECIFICATION OF THE POLYCHROHATOR

Dispersion system : Holocjraphic concave grating of radius

of curvature 1.0 m

Mounting type : Paschen-Runge mount.

Cratin'j frequency : 2160 grooves/ami

Angle of incidence : 31

Reciprocal linear dispersion : 4.62 A/uiin in 1s1. order

Resolution : 0.25 A

Wavelength range : 2288 A to 2813 A

Detection system : Phofcoraultiplier tube

Number of channels : 7

Wavelengths of the lines for

which exit cli.tr. are mounted : Cd 2288.0 A, Ni 2320.0 A, Co 2432.2 A,

B 2497.7 A, Mn 2605.7 A, Hg 2795.5 A,

and Cr 2843.2 A

7. APPLICATION

The polychromator will be used with inductively coupled plauna as

an excitation source for the detection of impurities Cd, Ni, Co, B,

Mn, Mg and Cr in uraniuu and other types of samples.

Acknowledgements : W<; lhank Shri S.S. Bhattachaxya for assistance in

designing the various mechanical assemblies and Shri H.B. Guhagarkar

for carrying out the fabrication of mechanical aounts used in this

project.

12

REFERENCES

1. V.A. Fassel, Science 201, 183(1978).

2. P.W.J.M. Boumanf; In "Kaission Spectror.copy' Ed. R.M. Earner.,

Dowden, Hulichinson and Ross, Inc., P..187.

3. R.M. Barnes, CRC Reviews in Anal. Cliera. 2, 203(1978).

4. S. Greenfield, The Spex Speaker 21, 111977).

5. V.A. Fassel and R.N. Knisdey, Anal. Chem. 1110A - 1120A

and 1150A - 1164A(1974).

6. M.I. Boulos, Pure and Appl. Chen. 51, 1321(1985).

7. S. Greenfield, H.Mc.D. McGeachin and P.O. SaiLh, TalanLa,

21, K1975); 21, 553(1975); 21, 1(1976).

B. J.P. Robin, Proy. analyl. atom. 5po.ct.rouc. 5_» 79(1982).

9. S. Greenfield, Proc. Analyt. Div. Chen. Soc, Sept., 279(1976).

10. Practical Spect.roscopy by H<irricon, Lord and Loufbourow,

Prentice Hall Inc. Enqlewood Cliffs, M.J., U.S.A.

11. Experimental Spc.ctroscopy by Sawyer, Pre.n1.ice Hall Inc.

Englewood Cliffs, N.J., U.S.A.

ROWLAND CIRCLE

\

\

\

FIG.1. PRINCIPLE OF THE CONCAUE CRATING.ROWLAND CIRCLE DIAMETER = RG : CONCAUE GRATING OF RADIUS OF CURVATURE RS : ENTRANCE SLITA B : SPECTRUM

HOLOGRAPHIC CONCAUEOIFFRACTION CRATING(2160 Croovcs / mm)

ROWLAND CIRCLE

ENTRANCE SLIT

CONDENSER LENS

OPLASMA TORCH

PM TUBE

PM TUBE

PM TUBEPM TUBE

FIG.'2. OPTICAL DIAGRAM OF A 1.0 M POLYCHRQMATOR TO BE USEDWITH INDUCflUELY COUPLED PLASMA EXCITATION SOURCE.

HOLOGRAPHIC CQNCAUEDIFFRACTION GRATING

G

FIG. 3. SCHEMATIC DIAGRAM OF ROULAHD CIRCLE, ENTRANCE AND EXITSLITS FOR THE CALCULATION Of THE EXIT SLI1 POSITIONS.

Fig. 4. ; tntrancc Slit Assembly

F-ISI 5 : (inning AssembK

ROWLAND CIRCLE EXIT SLIT

PM TUBE-

Fig. 6 ; Rowland Circle Mount

INCIDENTLIGHT

H. I".

PMT

D.C. AMPLIFIERrWWWWi

DliM

FIG.l. BLOCK DIAGRAM OF THE DETECTION SYSTEM.PMT : PHOTOMULTIPLIER TUBEH.T. : HIGH TENSION POWER SUPPLY UNITDWM : DIGITAL liOLTMETER

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