Physica B 151 (1988) 57-62 North-Holland, Amsterdam

NEUTRON I N T E R F E R O M E T R Y IN INDIA

V.C. R A K H E C H A , A .G . W A G H and B.A. D A S A N N A C H A R Y A Nuclear Physics Division, Bhabha Atomic Research Centre, Bombay 400 085, India

A spectrometer for carrying out perfect crystal diffractometry and neutron interferometry is proposed to be set up at the new 100 MW research reactor Dhruva, as part of a programme to expand and upgrade neutron beam based research activities at the Bhabha Atomic Research Centre (BARC). At present a facility has been set up at the CIRUS reactor where interference oscillations have been recorded using a symmetric LLL silicon interferometer. The setup at CIRUS will serve as a test bed for development of interferometry in BARC.

1. Introduction

A major effort is under way at the Bhabha Atomic Research Centre ( B A R C ) to substantial- ly expand and upgrade the neutron beam based research activities. A number of new facilities and several au tomated spectrometers are in the process of installation at the new 100 MW re- search reactor Dhruva, which became critical in 1985 and has since achieved 75% of its rated power level. The design of this reactor incorpo- rates several special features such as tangential beam holes, recessed cavities in the biological shield, cold and hot neutron sources and cold neutron guides. These features together with bet ter instrumentat ion are expected to provide significant gains for the proposed studies.

As part of this p rogramme a spect rometer for carrying out perfect-crystal diffractometry and interferometry will be set up at Dhruva. This spect rometer is planned to be installed on a cold neutron guide tube characterised by A*= 2.2 A. A schematic diagram of the proposed setup based on a double monochromator configuration is sketched in fig. 1. Several critical components of the spectrometer such as precision goniome- ters, translation stages and other manipulators are being fabricated in-house. Tests on the pro- posed vibration isolation system are being car- ried out.

Fabrication of monolithic silicon single-crystal LLL interferometers has also been taken up. Two dummy interferometers in the monolithic

LLL configuration were cut on a high-precision grinding machine, one f rom a borosilicate glass cylinder and the other f rom a low-grade silicon crystal. Dimensional accuracies of bet ter than 10 pLm were achieved during grinding operations on the dummies. A symmetric (220) LLL inter- fe rometer has recently been cut f rom a silicon single crystal ingot. It will be tested for interfer- ence oscillations after carrying out a controlled surface etching of its plates.

While this setup is taking shape, a neutron interferometer setup has been installed on a thermal neutron beam line of the 40 MW CIRUS reactor. This will initially serve as a test bed for neutron interferometry in BARC. With this setup, interferometric oscillations have been ob- served using an LLL interferometer ( IFM) loaned by Prof. H. Rauch. We shall now de-

MONOCHROMATOR

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VIBRATION FREE--I ~ ~ / ~--~,~/~/~ .EAYY ' S . I I1 I

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Fig. 1. Layout of the proposed interferometer setup at the Dhruva reactor on the cold neutron guide (A*= 2.2/~).

0378-4363/88/$03.50 t~ Elsevier Science Publishers B.V. (North-Hol land Physics Publishing Division)

58 V.C. Rakhecha et al. I Neutron interferometry in India

scribe the facility at C IRUS and present prelimi- nary results obtained with it.

2. Experimental setup at CIRUS

A sketch of the setup at CIRUS is shown in fig. 2. The spectrometer shares the mono- chromator drum of a triple-axis spectrometer . The monochromator crystal is mounted on a two-axis goniometer suspended in a cavity in the drum. The take-off angle (20M) has a fixed value of 90 ° for the monochromated beam. A pressed (111) silicon crystal with a mosaic of 35' and 1 0 m m thickness is used as a monochromator . Monochromatic neutrons of 1 .76A wavelength were obtained, without 1A contamination, using the (331) reflection. A 106 cm long wooden col- l imator lined inside with cadmium restricts the monochromat ic beam to an area of 8 m m (H) × 4 0 m m (V). A 1 0 m m diameter low-efficiency BF 3 detector mounted vertically on the col- l imator face monitors the monochromat ic beam intensity. The centre of the in terferometer table

is located 1 m downstream from the face of the collimator. The beam incident on the IFM has a wavelength spread of about 1% and an intensity of about 1000 neut rons /cm 2 s. Two 4 inch long He 3 detectors 4 0 m m in diameter filled to a pressure of 3 a tmospheres record the O- (for- ward) and H- (diffracted) neutron beam inten- sities emerging from the IFM. Each detector shield is mounted on a unislide to facilitate its translation transverse to the corresponding beam direction. The interferometer table has been suitably equipped as described in the next sec- tion to isolate the IFM from floor vibrations. The entire setup is enclosed inside a hut made of soft wood and plywood boards. The hut brings about a significant reduction in the acoustic noise inside.

3. IFM table assembly

Using a velocity-sensitive seismic sensor, reac- tor floor vibrations were measured at CIRUS. Peaks at several frequencies (--2 Hz, 10-12 Hz,

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NEUTRON INTERFEROMETER SET UP AT CIRUS

Fig. 2. Layout of the present interferometer setup at the CIRUS reactor on a thermal neutron beam line.

V.C. Rakhecha et al. / Neutron interferornetry in India 59

24 Hz, etc.) and their harmonics were observed in the noise spectrum. After several empirical trials the arrangement shown in fig. 3 was arrived at for vibration isolation of IFM. The individual components numbered in fig. 3 are described below in the same order.

(1) IFM resting freely on three soft felt pads glued onto a silicon plate. The silicon plate is supported on a small air pillow stabilized by the weight of a 9mm thick lead block of 55 x 100 mm 2 area.

(2) Arc goniometer for adjusting the orienta- tion of the H vector.

(3) Two translations in the horizontal plane, one along H and the other transverse to it.

(4) Rotary goniometer with an angular resolu- tion of 0.9". The phase plate rotation assembly is mounted directly on the rotary goniometer and is driven by a step motor through a 30 : 1 reduction gear.

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(~) 2 ~X 2 ~ 130 KG

Fig. 3. Schematic sketch of the arrangement used for vibra- tion isolation of the interferometer.

(5) Thermocol enclosure around the goniome- ter and IFM with sealed windows for passage of neutrons and for viewing. It is fully covered at the top. This helps to minimise temperature gradients across the IFM and to eliminate air currents.

(6), (9), (13), Layers of rubberized coir sheets. These are quite effective in vibration isolation and damping.

(7) Soft felt pad lining under thermocol box. (8) 2ft square, 130 kg granite slab. This serves

as a platform for mounting the goniometer as- sembly.

(10) Square aluminium support plates. The stability of the IFM can be further improved by placing lead bricks on the lower plate.

(11) Alternating layers of rubberised coir and soft rubber sheets sandwiched between MS an- gles for vibration isolation in the horizontal plane. Four such "springs" are mounted on the four sides.

(12) 4 soft bellows-covered metal springs mounted at the corners of the square plates (10), for isolation of low-frequency ( -1 Hz) vertical vibrations

(14) Solid rubber tyre for support and effec- tive isolation of high frequency vibrations.

(15) Circular aluminium support plate. (16) 3 wooden spacers at 120 ° spacing. (17) 3 iron pillars. (18) 20 mm thick iron ring with a central hole

to facilitate grouting on the reactor floor of the rotation axis for the H-detector. This ring is anchored to the floor through thick rubber washers.

(19) 1 inch thick rubber sheet ring between the iron ring and reactor floor, also effective in isolating high-frequency (>- -20Hz) vibrations

(20) Axis of rotation for the H-detector. (21) 4 oil dampers for damping stray vibra-

tions of the granite table, comprising aluminium fins rigidly attached to the top aluminium plate (10) and dipping in oil cans placed on the aluminium plate (15).

The natural resonance frequency of the entire setup is about 1.3 Hz for vertical and around 2 Hz for horizontal vibrations. Vibration isola- tion factors of about 10 at the resonance fre-

60 V.C. Rakhecha et al. / Neutron interferometry in India

quency and 50 to 100 and higher at higher fre- quencies have been measured.

4. Measurements at CIRUS

The first neutron interferometric measurement by amplitude division was reported [1] thirteen years ago. The working principle of a neutron LLL interferometer is well documented in the literature [2]. A monochromatic neutron beam incident at 0 B relative to the reflecting planes splits coherently into two partial beams after passing through the first plate of the IFM. The two partial beams get partially reflected at the second plate, recombine at the third plate and exit the IFM as two beams, one in the forward (D) and the other in the diffracted (H) direction. A phase difference can be introduced between the two partial beams by rotating a parallel faced plate about an axis normal to the plane of dif- fraction, in one of the gaps of the IFM. The corresponding variation in the path difference x between the two partial beams within the phase plate causes sinusoidal oscillation in the O- and H-beam intensities. The O and H intensities oscillate out of phase with each other conserving their sum and can be represented by

Io = Io + l ob + A cos(27rx/ta + q~o) ,

and

I . = ir~ + Irm - A cos(2~rx/ta + ~o) •

Here t A = 2~r /NAb c denotes A-thickness, N the atomic density and b c the neutron coherent scat- tering length of the phase plate material. The O- and H-beam interference contrast are measured by the ratios A / i o and A / i n , respectively, loB and IHB stand for the background intensities which are determined by recording the inten- sities of the IFM reflection and are subtracted out from the interference pattern.

The monolithic IFM used in our measure- ments was cut at the Atominstitut at Vienna in Prof. Rauch's laboratory and tested at ILL, Gre- noble. This IFM has been cut from a 75 mm long

perfect silicon crystal of 60 mm diameter in a ttt symmetric (220) LLL configuration. The nomi- nal distance between the plates is 28 mm, their thickness 3 mm and the maximum height from the base is 35 ram. The thickness of the IFM base below the plates is 14 mm.

The interferometer hut had to be heavily shiel- ded from fast neutrons to reduce the O and H backgrounds to the acceptable levels of around 0.1counts/s. The measured I o and i H values range typically around 0.5 and 1 count/s respec- tively.

During our preliminary measurements, phase plates made of commercial aluminium have been used. The interference patterns were recorded by monitoring the O- and H-beam intensities at regular increments in the phase plate orien- tation.

The data from one of the early runs with a 4.97mm thick phase plate is plotted in fig. 4.

E 0

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o

0

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-r

700

600

500

160(

1500

1400

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o o o o o o o

o o o

o o o

o o

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lJ t L , I , l J t -300 -200 -100 0 ÷ 100

PATH DIFFERENCE (MICRONS)

Fig. 4. The measured O, H and ( H - O ) / ( H + O) oscilla- tions for 1.76 .A, neutrons. The smooth curves through the data points represent least-squares fits. The beam size was 10 m m (H) × 21.5 m m (V).

V.C. Rakhecha et al. / Neutron interferometry in India 61

-[ o

z

0

o

700 [i , I l

0

o

I,--

z

0

A

O 4- -I-

o I -i-

-600 -400 -200 PATH DIFFERIrNCE (MICRONS)

Fig. 5. The interference pattern measured with a 10 x 10 mm neutron beam incident near the interferometer top.

The O and H intensity oscillations of identical periods, similar amplitudes and nearly out of phase with each other are clearly visible. The contrasts obtained for the O- and H- beams are about 11% and 4.4%, respectively. The fitted A-thickness, however, is smaller by about 15% than the expected value of 172 txm. The dis- crepancy was traced to the inaccuracies in the phase plate rotation mechanism. During these measurements the incident neutron beam illumi- nated an area of 10 mm (H) x 21.5 mm (V) near the top part of the IFM. Similar interference contrasts could be achieved by translating the IFM along H over a distance of 8 mm.

Results of measurements on the lower part of the IFM with a beam size of 10 mm (H) × 18 mm (V) were not satisfactory. Interference contrasts of only 4-5% for the O-beam and practically

zero for the H-beam were measured with 1.76/~ and 1.54/~ neutrons during such runs.

The best data so far has been recorded with a 10 x 10mm beam on the top part of the IFM using a 4.68 mm thick phase plate. These results, depicted in fig. 5, correspond to an O-beam contrast of about 30% and an H-beam contrast of 9%. The O-beam contrast compares favour- ably with the value of 28.6% obtained during a test run with the same IFM at ILL, Grenoble by Prof. Rauch. The O and H intensities are seen to oscillate exactly out of phase with each other with identical periods and amplitudes. The A- thickness of 180p~m obtained from a least- squares fit to this measurement is much closer to the expected value than before. As regards the constancy of ~00 from one cycle to another in the data however, there is a good scope for im- provement.

5. Future plans

From our experience so far a number of im- provements in the CIRUS setup are planned so as to streamline the interferometric measure- ments and enable accurate determination of co- herent scattering lengths. The improved setup will be calibrated by measuring several standard scattering lengths. Investigations in solid state physics and material science will then be taken up. The CIRUS facility will also be used to test interferometers fabricated in BARC for use at the proposed facility at Dhruva.

Acknowledgements

The authors are grateful to Prof. H. Rauch of the Austrian University, Vienna for the loan of a tested interferometer and soft springs as well as for the whole-hearted support extended by him to the neutron interferometer activity at BARC. Special thanks are due to his colleague Dr. J. Summhammer who spent four weeks in BARC and helped the authors record their first neutron interference pattern within this short period. The help from R.I.K. Moorthy of RED and V.K.

62 V.C. Rakhecha et al. / Neutron interferometry in India

Jain of the Seismology Section in vibration mea- surements is gratefully acknowledged. It is a pleasure to thank Dr. K.R. Rao for his support and encouragement throughout the course of this work. We also thank the Nuclear Physics Divi- sion Workshop for their prompt technical support.

References

[1] H. Rauch, W. Treimer and U. Bonse, Phys. Lett. 47 A (1974) 369.

[2] See eg., U. Bonse and H. Rauch, eds., Neutron Inter- ferometry (Clarendon Press, Oxford, 1979).

DISCUSSION

(Q) S .A . Werner: Could you tell us something about your new reactor?

(A) V.C. Rakhecha: Our new reactor is the fifth in the chain of research reactors at Trombay. It is a natural uranium fuelled, heavy-water-moderated and heavy-water-cooled thermal research reactor designed for neutron beam re- search, neutron activation, isotope production and reactor engineering experiments. At its maximum rated power level of 100 MW the maximum thermal neutron flux is expected to

be 2 × 10 TM neutrons/cm 2 s. Besides several radial beam tubes in the pile block there are four tangential beam tubes, two through tubes and beam tubes to insert hot and cold neutron sources. The design also incorporates built-in recessed cavities and cutaways in the biological shielding to enable closer access to the high flux region. Two guide tubes begin- ning at the calandria end get terminated outside the reactor hall in the "guide tube laboratory".