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DEVELOPMENT OF NEUTRON AND X-RAY DETECTORS

AND NEUTRON RADIOGRAPHY

AT BARC

A.M. Shaikh

Solid State Physics Division

Bhabha Atomic Research Centre

The author is the recipient of the DAE Technical ExcellenceAward for the year 2004.

A b s t r a c t

Design and development of neutron and X-ray detectors and R&D work in Neutron Radiography (NR) for non-

destructive evaluation are the important parts of the neutron beam and allied research programme of the

Solid State Physics Division (SSPD) of BARC. The detectors fabricated in the division not only meet the in-house

requirement of neutron spectrometers but also the need of other divisions in BARC, DAE units and some

universities and research institutes in India and abroad for a variety of applications. The NR facility set up by

SSPD at Apsara reactor has been used for a variety of applications in nuclear, aerospace, defense and

metallurgical industries. The work done in development of neutron and X-ray detectors and Neutron

Radiography since 1992 is reported in this article.

Neutron and X-ray Detectors

Radiation detectors play an important role in

medicine, biology, materials science and high-energy

physics for monitoring and imaging applications. Gas

filled detector, semiconductor detector and scintillation

detector are widely used in flux measurement, area

monitoring and scattering experiment applications with

each of them having their own advantages and

limitations. The detector technology is rapidly evolving

by making use of recent developments in material

processing, new detector designs, data acquisition and

analysis systems [1]. In case of neutron scattering

experiments the neutron beam intensities are low due to

collimation, monochromatisation and scattering.

Efficient neutron detection or imaging is therefore

essential to use the neutron beam time judicially. Demand

for neutron detectors with higher count rates, larger

scanning angles and finer position resolutions, is ever

increasing.

Among the various types of detectors, gas-filled detectors

are widely used especially by the neutron scattering

communities around the world. Gas-filled position

sensitive detectors [2] are conveniently used in various

spectrometers to scan large angles as these detectors

can be fabricated with large size and show high detection

efficiency. They have the advantages of

9 I s s u e n o . 2 7 3 O c t o b e r 2 0 0 6

†Detector type nomenclature: the number before alphabet indicates length of detectorin inches and A, B, C, D and E indicate cathode diameter of 0.4”, 0.5”, 1”, 1.5” and 2”

respectively. The anode is made of tungsten wire of 25 mm diameter.Cathode material: brass.

low gamma sensitivity, high neutron efficiency, very high

noiseless internal amplification, no radiation damage,

flexibility of size and fill gas pressure and need simple

counting and pulse processing electronics. Large area

position sensitive detectors with multiwire geometry

developed in 1970s have become widely popular for

many applications [3]. At Solid State Physics Division,

we are involved in indigenous development of gas filled

signal detectors and position sensitive detectors (PSDs)

for X-rays and neutrons for various applications at BARC

and other laboratories in India. These detectors are widely

used as low sensitivity monitor counters to very high

sensitivity signal detectors. Continuous efforts are put in

towards modification of detection techniques to carry

out the experiments efficiently. Various types of detectors

developed are 3He and BF3 filled detectors for neutrons,

linear 1-D single anode PSD, 1-D and 2-D multiwire PSDs,

curvilinear PSD and a microstrip based PSD for both

neutrons and X-rays[4-11]. These detectors show excellent

operational characteristics and stability over the long

periods. Various types of neutron proportional counters

fabricated in our laboratory are listed in Table 1 along

with their specifications and applications. Fig.1 shows

some of these detectors.

Table 2 gives photographs and salient features and

applications of various types of position sensitive detectors

designed, developed and successfully tested in our

laboratory. Many of the 1-D position sensitive neutron

detectors are mounted on the neutron spectrometers at

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Dhruva and working over the years satisfactorily.

The gas filled neutron proportional counters are

extensively used in BARC over a wide range of application

such as flux and area monitoring, spent fuel activity

measurement, study of nuclear reactions, measurement

residual activity of nuclear waste, measurement

of cosmic neutron radiations and

Fig.1: Some of the neutron proportional counters mentioned in Table 1

Table 2: Various types of Position Sensitive Detectors for Neutron and X-rays developed

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Table 2 contd...

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material studies using neutron scattering techniques.

The detectors are also supplied to atomic energy

establishments of other countries like Bangladesh,

Vietnam and Austria ( IAEA). Table 3 gives the statistics

about different types of detectors designed, fabricated

and supplied to various users in BARC and outside

institutions. Users other than BARC can procure these

detectors through the Technolgy Transfer & Collaboration

Division of BARC.

Neutron Radiography Facility and

Applications

The property of thermal neutrons, which makes them

valuable for studying industrial components is their high

penetration through widely used industrial materials such

Table 2 contd...

Table 2 contd...

Table 2 contd...

13 I s s u e n o . 2 7 3 O c t o b e r 2 0 0 6

as steel, aluminium or zirconium. Neutrons are efficiently

attenuated by only a few specific elements such as

hydrogen, boron, cadmium, samarium and gadolinium.

For example, organic materials or water attenuate

neutrons because of their high hydrogen content, while

many structural materials such as aluminium or steel are

nearly transparent. Neutron Radiography (NR) is a non-

destructive imaging technique for material testing. It is

similar to X-ray and gamma radiography and has some

special advantages in Nuclear, Aerospace, Ordnance and

rubber and plastic industries. Information is obtained on

the structure and inner processes of the object under

investigation by means of transmission. At BARC, neutron

radiography has been actively pursued by the Solid State

Physics Division for various applications, using the 400kW

swimming pool type reactor, Apsara as the neutron

source. In this facility the thermal neutrons from the

reactor are collimated by a divergent, cadmium lined

aluminum collimator with a length/inner diameter (L/D)

ratio of 90. A cadmium shutter facilitates the opening

and closing of the beam. The specimen can be mounted

about 60 cm from the collimator followed by a cassette

containing neutron converter and X-ray film. The whole

setup is properly shielded to avoid any radiation exposure

to the operator. Table 4 gives the important parameters

of the facility, which is schematically shown in Fig. 2.

Table 3 : Statistics of Neutron & X-ray Detectors Fabricated and Supplied

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The NR facility of Apsara has been used for a variety of

applications in nuclear, aerospace, defense and

metallurgical industries[12]. The facility has been

extensively used for recording neutron radiographs of

. experimental fuel elements,

. water contamination in marker shell loaded with

phosphorous,

. electric detonators,

Fig. 2 : Schematic of NR Facility at Apsara Reactor

15 I s s u e n o . 2 7 3 O c t o b e r 2 0 0 6

. satellite cable cutters and pyro valves,

. boron-aluminium composites,

. hydride blisters in irradiated zircaloy pressure

tubes,

. variety of hydrogenous and non hydrogenous

materials and

. two phase flows in metallic pipes.

While some of the radiographs are shown in Fig.3, the

most recent work on NR is described in the following

paragraphs.

Assessment of hydriding on

Zircaloy-2/Zr-Nb2.5% pressure tube

material using Neutron

Radiography

One of the most important applications of NR in the

nuclear field is the post irradiation examination of pressure

tube (PT) to check formation of hydride blisters if any. To

ascertain the detection of hydride blisters in zircaloy

pressure tubes, detectability limit of hydride was first

established[13]. For this purpose hydride blisters were

Fig. 3 : Neutron Radiographs of various objects taken with Apsara NR facility.

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created in the laboratory on hydrogen charged Zircaloy

pressure tubes under thermal gradient are examined using

neutron radiography. It was established that a zirconium

hydride blister of nearly 0.25% of job thickness could be

detected using neutron radiography. The theoretical

detectability was also analyzed and found to be in good

agreement with the experimental results. This work served

as reference to examine an irradiated pressure tube from

a power reactor[14,15,16]. Neutron radiographs of an

irradiated pressure tube [Rajasthan Atomic Power Station

(RAPS#2, K-7 position, 8.25 EFPY) sample were recorded

using Apsara NR facility after modifying it for handling

the radioactive zircaloy-2 pressure tube coupons. The PT

sample of size 59mm x 29mm x 4.2mm was radiographed

using transfer technique with 100m thick Dysprosium

converter screen. A PT strip containing four laboratory

generated hydride blisters were also mounted on the

same converter screen cassette to act as a reference.

Fig.4(a) shows neutron radiographs of laboratory

generated hydride blisters in zircaloy-2 and Zr-2.5% Nb

pressure tube coupons where as Fig. 4(b) shows hydride

blister streaks in the zircaloy-2 coupon of the pressure

tube from power reactor RAPS#2. Enlarged view of one

of the blisters is also shown to the right of the figure.

Neutron radiography was also used to study size and

shape of the zirconium hydride blister in the zircaloy-2

pressure tube. Figs. 5(a) and (b) show neutron

radiographs of a pressure tube with three laboratory

generated hydride blisters and with neutron beam incident

parallel and normal to the plane of the blisters

respectively. Fig.5 (a) shows lenticular shape of the blister

with nearly 2/3 of the blister embedded in the wall of

the tube. In the present photograph maximum width of

the blister corresponds to 1.5 mm in 4 mm thick wall of

the pressure tube. However, the smallest blister grown

in the laboratory was found to be mainly on the outer

surface of the pressure tube with almost no penetration

in the wall of pressure tube.

Fig. 4(a) : Hydride blisters in zircaloy-2(L)and Zr-Nb(R) PT coupons (Lab. Generated)

Fig. 4(b) : Hydride blister streak in zircaloy-2 PTcoupon (L) from a power reactor. Enlarged view

of the blister shown in (R)

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Recently characterization of hydride blisters in burst tested

Zr-2.5% Nb pressure tube was done using Apsara NR

facility [17]. Hydride blisters were grown in a 16.5 cm

long Zr-2.5% Nb pressure tube piece in RED, BARC. The

tube was subjected to burst testing. The tube failed axially

in brittle manner during testing. The axial crack passed

through two hydride blisters which was also seen in the

neutron radiograph (Fig. 6).

HYSEN radiography technique with

NR facility at Apsara reactor

The technique, referred as Hydrogen Sensitive Epithermal

Neutron (HYSEN) radiography technique was first

developed for imaging small amounts of hydrogenous

materials encapsulated within high thermal neutron

absorbers, and found to be useful in study of hydride-

induced embrittlement of metals. The HYSEN imaging

system consists of a converter screen (In) and a neutron

beam filter ( In + Cd ) with the object placed between

the screen and filter. For neutrons, In has a resonance

peak at 1.49 eV. Combination of Cd foil with In foil

almost completely cuts off neutrons with energy lower

than 1.49 eV. Incident neutrons with energy higher than

1.49 eV, which pass through, are scattered elastically by

hydrogen atoms present in the object. The neutrons that

are slowed down to the vicinity of 1.49 eV by this

scattering are absorbed by the second In foil placed

behind the sample. Since the image is induced by the

scattered neutrons, the resolution and the sensitivity of

the technique are highly dependent on the object

dimensions and object to screen distance. Hydrogen

concentrations as low as 50 ppm ( 0.020 mg H /cm2 ) in

0.62 mm zircaloy coupons have been reported. The

detection limit of standard NR techniques is about 0.66

mg H/cm2. Thus the sensitivity of this method was found

to be about 30 times better than the normal NR

techniques.

Setting up of HYSEN radiography technique with existing

NR facility at Apsara reactor was undertaken as a part of

coordinated research project of IAEA[15,18]. The imaging

Fig. 5 : NR of zircaloy-2 PT coupons withneutrons incident parallel (a)

and perpendicular(b) tothe plane of blisters

Fig. 6 : Neutron radiograph of Zr-2.5%Nbpressure tube piece containing laboratory

generated hydride blisters(B).The axial crack isseen passing through two blisters

at position 4 on the tube.

(a) (b)

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system (Fig.7) consisted of a 250 mm indium converter

screen and a neutron beam filter comprised of 1 mm

cadmium and 1 mm indium foils, with the object being

placed between the filter and screen. The system was

exposed to neutron beam of the NR facility. Neutron

radiograph of 1-, 2-, 3-, 4- layers of cellophane adhesive

tape as object was recorded. The gradation of hydrogen

concentration in successive layers of adhesive tapes was

clearly seen in the radiograph (Fig.8). At present the

experimental set up is suitable for recording HYSEN

photographs of very thin hydrogenous samples. The

results of preliminary measurements are regarded as

encouraging, considering the very simple system that

was used (Fig. 9). The work on improvement over the

neutron images on a video monitor. This method is

especially attractive for the high-throughput inspection

of parts requiring instant feedback of information.

Unfortunately, it has limitations in its applications. It

cannot be used for inspection of the irradiated specimens

because of its sensitivity to gamma radiation. Typically,

an electronic imaging method utilizes a fluorescent

converter screen to produce a light signal that is suitably

Fig. 7 : Schematic of basic HYSENRadiography system

Fig. 8 : Neutron radiograph of 1-, 2-,3-, 4- layers of cellophane adhesive tape

as an object

Fig. 9 : Sensitivity of 8 mg/cm2 correspondingto 1mg/cm2 of hydrogen

existing set up with different neutron detector and filter

foils and use of electronic imaging camera for recording

the radiograph is in progress.

Digital Neutron Imaging

One of the important radiography methods is Electronic

Imaging or Real Time Radiography, which produces

19 I s s u e n o . 2 7 3 O c t o b e r 2 0 0 6

amplified and fed into video equipment for viewing.

The video output is digitized using frame grabber card

and processed using an onboard processor. Image

acquisition software performs operations like online

thresholding, contrast stretching, integration, averaging

etc. Trained human eye can distinguish 50 to 100 gray

levels between black and white.

Digital imaging accelerates this

probing dramatically, up to 4096

gray levels are differentiated. Use of

planar detectors like low light

cameras (e.g. CCD, C-CCD…) in

combination with fluorescent screens

is made to record the radiographs.

An electronic imaging system

(Figs. 10 and 11) based on

commercially available image

intensifier tube and low cost CCD

camera has been developed and

tested [19]. The neutron beam

transmitted through the sample is

absorbed in a NE-426 scintillator

screen. The light produced by the

screen is reflected by 90° and focused

onto the input fibre optic face of an

image intensifier tube. The output

image is focused onto a CCD camera

using, an F#1.4 lens. The CCD

camera used has 756(H) x 581(V)

pixel array and image intensifier used

has 30 lp/mm resolution with gain

of 105 Cd/m2/lx.The video output of

CCD is digitised using a frame grabber card and processed

using onboard processor. Various types of assemblies

and components were scanned using the imaging system.

Neutron images were instantly seen on the video monitor

indicating that flux at Apsara NR facility was adequate

for real time imaging work. Fig.12 shows images of some

of the objects scanned using the imaging system.

For Neutron Tomography the sample to be imaged is

placed on a platform, which is rotated using a stepper

motor. The stepper motor is controlled using a PC based

controller card. The sample rotation, image acquisition

and processing have been interfaced together within a

single control program. The images were recorded in

various angles and used in reconstruction program based

on Convolution Back Program method for obtaining CT

Fig. 10 : Schematic diagram of electronicimaging setup at Apsara

Fig. 11 : Photograph of an electronic imaging system usedat Apsara for real time neutron radiography

I s s u e n o . 2 7 3 O c t o b e r 2 0 0 620

scans at various slices. Several sets of samples were

fabricated to assess the quality of tomography system

[19,20]. One such sample is a cylindrical brass rod of 40

mm diameter with holes ranging from 1 mm to 5 mm,

some of them filled with wax, air and Indium wire, is

shown in Fig.13. Fig.14 shows the CT scan of this

sample. All the holes either wax or air filled, are visible in

the image.

Visualisation and analysis of water/air,

water/vapour two phase flow inside

metallic pipes under high temperature

and pressure condition is of

considerable importance in thermal

hydraulic design of nuclear reactors.

It is important to know under what

condition bubbly to slug or slug to

annular flow transitions occur. Ideally,

one would like to have method where

one not only determines such

transitions quantitatively but also

visualize the flow pattern. Neutrons

provide this facility. Most of the

cladding metals such as aluminum or steel are transparent

to neutrons whereas hydrogenous materials such as water

are relatively opaque. This makes neutrons a unique tool

for study of flow of water inside metallic pipes. Such

two phase flow visualization is done using electronic

imaging technique [21]. The prototype electronic

imaging system developed by us has been used for

Fig.12 : Neutron images of (1) carburettor, (2) spark plug,(3) INSAT cable cutter & U-tube filled with wax recorded

using electronic imaging system at Apsara

Fig.13 : Schematic diagram of a sample made ofsolid brass (40 mm φ) with holes of 1mm to 5 mm filled

with wax, air and indium wire

21 I s s u e n o . 2 7 3 O c t o b e r 2 0 0 6

visualization of two phase flow (water/air) inside metallic

pipes. Simulation of bubbly, slug and annular flow was

done using various combinations of flow rates of water

and air using the experimental arrangement shown in

Fig.15. The transition from one flow regime to other has

been visualized (Fig.16). Aluminum pipes of diameter

from 12 mm to 25 mm and wall thickness of 1.5 mm to

2 mm, and steel pipe of 12 mm diameter and 1.5 mm

wall thickness have been used for visualization purpose.

Acknowledgement

I thank my colleagues Mrs. S. S. Desai, Mr. A.K. Patra,

Mr. J.N. Joshi, late Mr. N.D. Kalikar, Mrs. M. Padalakshmi,

Mr. A.G. Temkar and Mr. S.M. Patkar for their sincere,

valuable assistance and hard work in fabrication and

testing of neutron and X-ray detectors. Thanks are also

due to Prof. K. Rajanna, Indian Institute of Science,

Bangalore for collaboration in microstrip detector work.

The work in Neutron Radiography was performed in

association with various research groups in BARC. I thank

Mr. B.K. Shah and Dr. P.R. Vaidya (AFD), Mr. K.C. Sahoo,

Mr.D.N. Sah, Mr. S. Gangotra (PIED) for their interest in

investigations of zirconium hydride

Fig.14 : Neutron computed tomographicimage of sample shown above

Fig. 15 : Experimental setup for two phase flow study at Apsara reactor NR Facility

I s s u e n o . 2 7 3 O c t o b e r 2 0 0 622

2. Position Sensitive Detection for Thermal Neutrons,

Edited by P. Convert and J.B., Forsyth Academy

press London.

3. G. Charpak, R.Bouclier, T. Bressani, J. Favier and C.

Zupanicic, Nucl. Instru. and Meth. 62(1968) 262.

4. Neutron Detection Techniques in Neutron Scattering

and Radiography Experiments, A.M. Shaikh, Bulletin

of Indian Association of Nuclear Chemists and Allied

Scientists Vol.1, No.3 , 66-71, 2002 .

5. Development of Gas Filled Radiation Detectors for

X-rays and Neutrons at SSPD, BARC, S.S. Desai and

A.M. Shaikh, Proceedings of DAE-BRNS National

Symposium on Compact Nuclear Instruments and

Radiation Detectors-2005, 495-502, 2005.

6. Two dimensional position sensitive neutron detector,

A.M. Shaikh, S.S. Desai, and A.K. Patra, Pramana-J

of Physics, 63(2), 465-470 (2004).

7. Development of a microstrip gas chamber to study

the effect of drift gap on its performance,

Fig.16 : Single frame image of water/air flow inside aluminium pipe at twodifferent instants

blisters and Dr. Amar Sinha (HPPD) for the work on

electronic imaging. My thanks are due to Dr. K.R. Rao

(Ex-Director, Solid State and Spectroscopy Group, BARC)

for his interest, suggestions and many useful discussions

in development of neutron detectors and neutron

radiography. I acknowledge support of Dr. A. Sequiera

and Dr. M. Ramanadham (Ex- Heads of SSPD) and Dr.

S.L. Chaplot, Head, SSPD and for their keen interest in

the work. I am indebted to Dr. Anil Kakodkar, Chairman

AEC and Dr. Srikumar Banerjee, Director BARC for the

recognition of my work with DAE award for Technical

Excellence for the year 2004.

References

1. Knoll G. F. Knoll, Radiation Detection and

Measurement, 2nd edition John Wiley and Sons,

1989.

23 I s s u e n o . 2 7 3 O c t o b e r 2 0 0 6

V. Radhakrishna, K. Rajanna, S.S. Desai and A.M.

Shaikh, Rev. Sci. Instr., 72( 2 ), 1361, 2001.

8. Development and Studies on Microstrip detector, V.

Radhakrishna, S.S. Desai, A.M. Shaikh and K.

Rajanna, Current Science (2003) vol. 84, No.10,

2003, pp 1327.

9. Study on meandering resistive strip for microstrip

position sensitive detector using charge division

method, V. Radhakrishna, K. Rajanna, S.S. Desai and

A.M. Shaikh, Rev. Sci. Instr.,74(10), 4263-

4267(2003).

10. Development of a microstrip based neutron detector,

S.S. Desai, and A.M. Shaikh, V. Radhakrishna and

K. Rajanna, Pramana-J of Physics, Vol. 63(2), 465-

470 (2004).

11. A Curvilinear Position Sensitive Neutron Detector,

S.S. Desai and A.M. Shaikh, Solid State Physics (India)

Vol. 50, 315(2005).

12. Y.D. Dande et al, Ind. J. Pure and Appl. Phys.,

29(1991)721.

13. Detection of zirconium hydride blister in zircaloy

tubes using neutron radiography, A.M. Shaikh, P.R.

Vaidya, B.K. Shah and P.G. Kulkarni, International

Seminar on Developments on Enhancement of

Research Reactor Utilization, IAEA-SR-198/26, 1996.

14. Neutron Radiography of contact location of

irradiated zircaloy-2 pressure tube from RAPS#2, S.

Gangotra, P.M. Ouseph, A.M. Shaikh and K.C.

Sahoo, in Trends in NDE science and Technology,

proceedings of the 14th WCNDT, Vol. 3, 1453-1456,

1996.

15. Bulk Hydrogen Analysis using Neutrons, A.M. Shaikh

et al, IAEA-CRP Report F1-RC-655.3, 2003.

16. Application of Neutron Radiography and Neutron

Diffraction techniques in study of zirconium hydride

blisters in zirconium based pressure tube materials”,

A.M. Shaikh, P.R. Vaidya, B.K. Shah, S. Gangotra

and K.C. Sahoo, J. of Non Destructive Testing and

Evaluation, Vol.2(3), 9-12, 2003.

17. Study of hydride blisters grown on Zr-2.5Nb pressure

tube spool piece under stimulated condition of in-

reactor pressure and temperature, S.K. Sinha, D.N.

Shah, Suparna Banerjee, V.K. Suri, K. Gurumurthy,

A.M. Shaikh and R.K. Sinha, BARC Report, 2006.

18. Bulk Hydrogen analysis using Neutrons: Hydrogen

Sensitive Epithermal Neutron Radiography Technique

(HYSEN), A.M. Shaikh, Proceedings of National

Seminar on Non-Destructive Evaluation, 155-157,

2003.

19. Development of CCD/Image intensifier based 2D/

3D tomography system and its application in neutron

and gamma tomography, Amar Sinha, A.M. Shaikh,

A. Shyam , Nuclear Instruments and Methods in

Physics Research B142, 425-431(1998).

20. Development of CCD/Image intensifier based 2D/

3D tomography system and its application in

neutron and gamma tomography, Amar Sinha, A.M.

Shaikh, A. Shyam and L.V. Kulkarni, Proceedings

of Twelfth National Symposium on Radiation

Physics, 142-144(1998).

21. Use of neutrons for visualization of water/air flow

inside metallic pipes, Amar Sinha, A.M. Shaikh, A.

Shyam and L.V. Kulkarni, Proceedings of Twelfth

National Symposium on Radiation Physics, 243-

245(1998).

I s s u e n o . 2 7 3 O c t o b e r 2 0 0 624

Dr. A.M. Shaikh joined BARC in 1992.

His areas of specialization are neutron

& X-ray detectors, neutron

radiography, high temperature X-ray

crystallography and instrumentation.

A Ph.D. from IISc, Bangalore, he was a

postdoctoral fellow at the University of

Washington Seattle, USA and has worked

as faculty member at IISc, Bangalore and

University of Kuwait. Dr. Shaikh has been associated with IAEA in

programmes related to Neutron Radiography and worked as an

IAEA expert in this field. He has about 90 research papers in national

and international journals and conference proceedings to his credit.

Dr. Shaikh is on the Board of Studies and Board of Ph.D. Examiners

of many universities and research institutions in India. Dr. Shaikh is

the recipient of DAE-Technical Excellence Award-2004 and also the

ISNT National Award-2003.