ABSTRACT This report describes the improvements made in the detection system of the pulse

radiolysis facility based on a 7 MeV Linear Electron Accelerator (LINAC) located in the

Radiation Chemistry & Chemical Dynamics Division of Bhabha Atomic Research

Centre. The facility was created in 1986 for kinetic studies of transient species whose

absorption lies between 200 and 700 nm. The newly developed detection circuits consist

of a silicon (Si) photodiode (PD) detector for the wavelength range 450 – 1100 nm and a

germanium (Ge) photodiode detector for the wavelength range 900-1600 nm. With these

photodiode-based detection set-up, kinetic experiments are now routinely carried out in

the wavelength range 450 – 1600 nm. The performance of these circuits has been tested

using standard chemical systems. The rise time has been found to be 150 ns. The photo-

multiplier tube (PMT) bleeder circuit has been modified. A new DC back-off circuit has

been built and installed in order to avoid droop at longer time scales. A steady baseline

upto10 s with PMT and upto100 s with PD is available without any droop. The RF

interference generated during the delivery of the electron beam pulse by the LINAC

normally limits the measurements below 20 mV signal. The introduction of a fibre optic

cable between the LINAC cave and the data acquisition laboratory has enabled us to

analyse even weak signals of the order of 5 mV.

IMPROVEMENTS IN DETECTION SYSTEM FOR PULSE RADIOLYSIS FACILITY

V.N. Rao, *R. Manimaran, M. Toley, S.J.Shinde, *K.G. Girija, *R.K. Mishra,

*S.A. Nadkarni, Hari Mohan, A.V. Sapre and T. Mukherjee

I. INTRODUCTION:

The technique of pulse radiolysis is a very powerful tool for the study of

mechanisms of fast reactions initiated by ionizing radiations in very short time durations

of the order of nanoseconds. Since its discovery in the early sixties, it has found

applications in gaining a better understanding of processes of relevance in diverse fields

such as radiobiology, radiation sensitization and protection, redox chemistry of inorganic

ions and organic molecules, photophysical properties of electronically excited species,

radiation damage studies of semiconductor devices, reactions concerned with the

degradation of materials in nuclear reactor environments, etc. A pulse radiolysis facility

has been installed in the Radiation Chemistry & Chemical Dynamics Division (formerly

known as Chemistry Division) of Bhabha Atomic Research Centre in 1986. In the first

phase, raw data were collected in the form of oscilloscope traces representing the

variation with time of voltage signals. Traces were transferred to a X-Y Recorder and

data were analyzed manually. This set-up has gone through various developmental

stages. At the first stage, as the manual processing of data generated in the form of

oscilloscope traces is cumbersome and time consuming, the oscilloscope has been

interfaced with an IBM compatible personal computer so that data is acquired and

analyzed in the computer. Details of this facility have been described elsewhere (Ref.1).

In this set-up, the photomultiplier tube (PMT R-955) is used as the light detector, which

has a nearly flat spectral response in the 200 – 700 nm wavelength region. In the next

stage, we endeavoured to have an experimental facility to study transient species whose

absorbance lies beyond 700 nm in the near infra red region, resulting in development of

two photodiode silicon (Si) & germanium (Ge) based detection systems with which one

can do the experiments from 500 nm to 1600 nm wavelength region. Also, the limitations

encountered while using this facility, i.e., radio frequency (RF) interference at the onset

of signals, which limits measurement of signals of the order of less than 20 mV and

droops at long duration measurements, have been eliminated (Ref.2).

This report describes the developmental work carried out to extend the

experimental facility of pulse radiolysis set-up upto 1600 nm, to eliminate RF noise

interference and to solve the droop problems at long time measurements.

II. PULSE RADIOLYSIS SET-UP:

The pulse radiolysis set-up is based on a linear electron accelerator (LINAC),

which is capable of giving single shot of 7 MeV energy electron beam pulses of widths

selectable from 5 – 500 ns, 2 µs, and corresponding peak currents of 1000 – 70 mA,

respectively. A schematic block diagram of the set-up is shown in Fig.1. In this

accelerator, the source of electrons to be accelerated are generated from a back-

bombarded red hot tungsten cathode and accelerated towards the anode which has a hole

at its centre by application of –43 KV of 2 µs pulses to the cathode with respect to the

anode. Thus, 2 µs pulses of electrons of 43 KeV energy are initially produced which are

focussed by electromagnetic lenses into a deflector chamber and then to a corrugated

cylindrical wave guide which is excited by 3 GHz, 1.8 MW peak power of RF field of

width 2 µs (produced by a Magnetron). The electrons from the gun entering in the correct

phase of the R.F. field are accelerated in vacuum (10-8 mbar achieved by using ion

pumps) to the energy of 7 MeV by the time they reach the other end of the waveguide.

The accelerated electrons are focussed by solenoid focussing coil to obtain a well-defined

uniform beam, which comes out of the waveguide through a thin titanium window. The

transient changes in the absorbance of the solution caused by the electron beam pulse are

monitored with the help of a collimated light beam from a 450 W xenon arc lamp. The

accelerator, sample cell and monitoring light source (xenon arc lamp) are housed in a

shielded cave and the monitoring light beam after passing through the sample cell is

directed to the detection room through a tunnel in the shield wall (1.5 metre thick

concrete) with the help of fused silica lenses and aluminium coated mirrors. The light

beam is finally focussed on to the entrance slit of a monochromator.

II.1. DETECTON SYSTEMS:

In any kind of spectrophotometry, photodetectors are used to produce an electrical

signal, usually a current, which is proportional to the light intensity. Here, PMT for the

spectral range of 200-700 nm, silicon photodiode for 500-1100 nm and germanium

photodiode for 900-1600 nm region are used. The photodetector is mounted at the exit

slit of the monochromator. The electrical signal from the photodetector is captured on a

100 MHz, 400 Msample s-1 L&T digital storage oscilloscope after the steady-state current

is compensated by a DC back-off circuitry. An electromechanically operated shutter

interposed between the monitoring light source and the sample cell is normally kept

closed to prevent photolysis of the sample and can be opened for stipulated duration,

which is selectable from 1 ms to 99 s. For monitoring the weak absorption signals lasting

for very short duration (fast transient measurements), the intensity of the xenon arc lamp

light is enhanced by about 20 – 100 times for a duration of around 3 ms, depending on

the wavelength region, to achieve improved signal to noise ratio. Normally the pulsed

light intensity is steady for around 100 – 150 µs, during which time the transient

absorption measurements are done accurately. For long duration measurements, the

boosting of the xenon arc lamp is kept OFF. Synchronization of the electron beam pulse

with the shutter opening, pulsing of the arc lamp and triggering of the baseline of the

oscilloscope are accomplished with the sequential delay pulse generator. The sequence of

triggering the various instruments is shown in Fig.2. Appropriate cutoff filters are placed

before the monochromator entrance slit to eliminate artifacts from second-order

diffracted light of shorter wavelengths. At any instant of time after the electron beam

pulse, a plot of absorbance versus wavelength reflects the absorption spectrum of the

species present in the sample cell at that instant of time. The signals captured in the

oscilloscope are transferred to a personal computer via IEE488 interface and processed in

the computer with an in-house programme.

II.1.1. PMT DETECTION SYSTEM:

A PMT consists of essentially a photo-cathode or primary emitting electrode, a

series of plates called dynodes which on impact by electrons emit secondary electrons

and multiply these up considerably, and finally an anode or collecting electrode, which

collects all the electrons. By connecting a resistor called load resistor (RL) at the

collecting electrode, the multiplied electron strength is monitored. A “Side-On” type

PMT (Model R-955, Hamamatsu, Japan) is used for pulse radiolysis experiments in

wavelength region 200-700 nm. Its small size is compatible with that of the rectangular

monochromator slit.

The specifications of the PMT are as follows:

Spectral Response : 160-930 nm with

peak at 400 nm

Uniform spectral response : 200-700 nm

Photo-cathode material : Multialkali

Window Material : Fused silica

Dynode structure : Circular cage with 9 dynodes

Socket : E678-11A

Max. Anode to cathode voltage : 1250 Vdc

Anode to last dynode voltage : 250 Vdc

Average anode current : 0.1 mA

Current amplification : 1.0 x 10 7

Anode dark current : 50 nA max

Rise time : 2.2 ns

Electron transit time : 22 ns

A 5-stage biasing circuit has been developed to use the PMT for fast transients of

few nanoseconds to slow reactions of the order of few seconds. The cathode is kept at

high negative potential with respect to the ground by a 50 – 1000 Volts DC/50mA

continuously variable regulated power supply (Model: 8330, Sairush Electronics

System). Fig.3 shows the PMT bleeder circuit for the pulse operation. In order to

maintain dynode potentials at a constant value during the pulse operations and obtain

high peak current, the zener diodes with capacitors are used. The output signal is drawn

from dynode-6. A 50 Ω coaxial cable is connected between the dynode and the load

resistor (RL).

In the earlier PMT detection circuit, the bleeder current was of the order of 1 mA,

which was very much suitable for fast transition (short duration) measurements from few

nanoseconds to 1 ms. For long duration measurements, the PMT operates as good as in

DC mode and the photocurrent becomes of the same order of magnitude as the divider

current which leads to a non-linearity in the PMT response. To overcome this problem, a

new PMT detection circuit has been developed, shown in Fig.3, which draws maximum

bleeder current of the order of 30 mA, with high wattage resistors. A cooling fan is

incorporated to avoid drift in the resistance due to overheating. With this new PMT

circuit and new back-off circuit (described below) the experiments can now be carried

out from short duration of the order of few nanoseconds to long duration of the order of

10 s.

II.1.2. PHOTODIODE DETECTION SYSTEM:

Semiconductor photodiodes (PD) are complementary to PMT. The photodiodes

require low voltage supplies, have spectral response from the visible to the near infra red

region, high quantum efficiency, lower noise and the ability to withstand very high peak

currents. Since there is no amplification in the photodiode, an external amplifier is used.

The photodiodes operate by the absorption of light photons or charged particles to

generate a flow of current in an external circuit. Photodiodes are used to detect the

presence or absence of minute quantities of light and are calibrated to measure the

intensity of light extremely accurately from intensities below 1 pW/cm2 to intensities

above 10 mW/cm2. Both the germanium and silicon PDs are operated in photoconductive

mode by applying reverse bias across the photodiodes. This results in a wider depletion

region, lower junction capacitance, lower series resistance, shorter rise time, and linear

response in photocurrent over a wide range of light intensity.

Since the PMT detection system has an upper limit on the samples whose

absorption lies beyond 750 nm, we have incorporated two photodiode (silicon and

germanium) based detection systems:

(i)A silicon photodiode based detection system (PDA55, Thorlab, USA) has the

spectral response from 450 nm – 1050 nm wavelength region. It consists of a silicon

photodiode, a switchable gain amplifier and a voltage to current converter. This system

can detect the light signals whose electrical bandwidth is from DC to 10 MHz. A buffered

output drives a 50 Ω input impedance up to 10 volts. The silicon detector responsivity is

shown in Fig. 4.

The specifications of this photodiode with built-in amplifier are as follows:

Detector : silicon photodiode

Active Area : 3.6 x 3.6 mm

Response : 320 nm to 1100 nm

Flat response : 450 nm to 1050 nm

Peak response : 0.6 A/W @ 960 nm

Bandwidth : DC to 10 MHz

Output voltage : 0 to 10 V

Gain steps : 0, 10, 20, 30, 40 dB

Gain switch : 5 position rotary switch

Operating temperature : -20o to 70o C

(ii) A preamplifier has been developed using fast operational amplifier LH0032

developed and installed along with a germanium photodiode to detect signals in the range

of 900 –1600 nm. The responsivity curve of the germanium detector is shown in Fig.5.

Specification of the op-amp:

Bandwidth : 70 MHz

Input bias current : 20 pA max

Supply voltage : ±18 V max

Input voltage : ± supply volts

Differential Input voltage : ±30 V

Supply current : 20 mA

Slew rate : 500 V/µs

Photodiode specifications:

Detector : Germanium

Active Area : 5 mm2

Flat response : 900 – 1600 nm

Peak response : 1400 nm

Voltage to Current Converter:

Basically both the PMT and PD deliver the current proportional to the light

intensity. However, for the same amount of light input, PMT, being current amplifier,

gives much larger current compared to PD. Hence, in case of PD, it is common practice

to use a preamplifier circuit to get a larger signal. Since the preamplifier delivers the

output in voltage form, and our back-off circuit requires current signal, a voltage to

current (V-I) converter is designed and installed. This circuit is shown in Fig. 6.

When the condition, R1 = R2

and R3 = R4 + R5 are met this circuit delivers the current given by

Iout = R3 x Vin / (R1 x R5).

In order to nullify the dark current present in the PD, an offset nulling is done at inverting

terminal of the op-amp by the potentiometer P1.

The op-amp used in this V-I converter is LF356. This op-amp is a wideband low

noise, low drift amplifier and is widely used as a photocell amplifier. It has the following

features:

• Low input bias current of 30 pA.

• Low input offset current of 3 pA.

• Low input offset voltage of 1 mV.

• JFET input impedance of about 1012 Ω

• Wide gain bandwidth of 5 MHz.

• Fast slew rate of 12 V/µs

Resistors R1, R2 and R3 have each been selected equal to 1 KΩ and R4 has been

selected 1 KΩ potentiometer to adjust the output current. Resistor R5 has been selected to

be 47 Ω.

II.2. MONOCHROMATOR:

Initially, a high intensity grating monochromator (Krotos Analytical Instruments

Inc, USA, Model GM 252, with Grating Model GMA 252-20) which has the spectral

range 180-800 nm, with dispersion of 3 nm/mm is used. Slit widths of the

monochromator are continuously variable from 0.01 - 6.0 mm and correspondingly give

bandwidths of 0.1 -19.8 nm. In this monochromator, due to the mechanical imperfections,

the scattered light is mixed with the chosen wavelength and leads to loss of efficiencies

leading to impurity of the transmitted light.

A new monochromator (Digikrom Model CM110), which has the spectral range

200 - 3000 nm, has now been installed. It is basically a 1/8th meter Ebert-Fastic

monochromator using spherical collimating and camera mirrors, and has 2 gratings.

Grating 1 covers the spectral range 200 - 750 nm and grating 2 covers the spectral range

800 - 3000 nm. This monochromator can be coupled with any computer via. RS232 serial

interface. The CM110 uses a novel digital drive. The previous monochromator used a

delicate mechanical sine generation mechanism to make wavelength linear with the

motor rotation. The CM110 performs this conversion with the software, resulting in two

advantages: i) It is much more rugged than sine drive monochromator and ii) It can

switch between two gratings by a simple rotation while sine drive monochromator

cannot. The CM110 is more efficient over a broad spectral range than concave

holographic grating monochromators. First, unlike the CM110, concave grating

monochromators only correct aberrations over about an octave of wavelength, normally

chosen to be 350 - 750 nm. Outside of this region the strong aberrations cause great light

loss. Second, holographic concave gratings generally have half the diffraction efficiency

of plane gratings because the groove profiles are not well determined. Third, because the

CM110 can use two gratings, good efficiency over a broad spectral range results. This is

not possible in today’s concave holographic grating monochromators.

II.3. DC BACK-OFF CIRCUIT:

Baseline restoration or “back-off” circuits have been used for a number of years

to permit the determination of small changes on a large signal to be made on two

measuring systems, one of which records the large signal while the other records the

small change in the signal. These arrangements have been used in pulse radiolysis optical

detection systems where kinetic information following a radiation pulse is recorded on an

oscilloscope or digitizer.

In the kinetic spectrophotometric technique used in the pulse radiolysis facility,

the measured signal is the transient intensity of an analysing light beam passing through a

chemical cell. For monitoring upto at least 90 % of the decay of the species, the system

should therefore be capable of sensing changes in voltages as low as 1%. In order to do

this, the background current signal due to the analysing light must be backed off to enable

sensitive detection of the super-imposed transient signal. The signal is usually presented

as the current output of a photodetector, which is then converted to a voltage signal,

processed and monitored on an oscilloscope. To maximise the signal to noise ratio of the

signal, the analyzing light level is made as large as possible within the constraint of the

linear operation of the detector. Basically the back-off technique involves a current

summation at the input impedance of a wideband oscilloscope connected directly to the

output of the detector.

When a pulsed light source is used, manual back-off is very inconvenient due to

light variation, hence an automatic back-off circuit is fabricated. This, in principle, is an

electronic circuit that provides a feedback, which generates the exact amount of the

current required to back-off the initial anode current, shown in Fig. 7.

. The photodetector current corresponding to the initial light intensity I0, produces a

signal at the input of the amplifier A1 which charges the capacitor C1, if switch S1 is

closed. This will drive amplifier A2, which generates a feedback current of the right

magnitude and opposite polarity to reduce the input signal to zero. The operation is

performed in the time constant of the circuit, which is 33 µs. The potential of the

capacitor C2, separated by a second switch S2, is a measure of the anode current. A

digital panel meter (DPM) is connected at the output of the amplifier A3, which is

calibrated to read I0 value.

When switches 1 & 2 are closed, the circuit provides the feed back loop that keeps

the input current at amplifier A1 to zero. Any variation in current occurring at the input is

automatically compensated in a few microseconds (33 µs). The DPM reads the steady

anode current I0. If S1 is opened first before the input signal varies due to light absorption,

the loop becomes inactive, but the potential of the capacitor C1 and zero anode potential

will be maintained. Any subsequent variation due to the absorption signal will be

displayed on the oscilloscope. In this condition, the oscilloscope may be used at

maximum sensitivity without any offset problem due to the steady light. The time of

opening and closing S1 is controlled by the sweep gate of the scope itself or by an

independent signal gate synchronized with the event of interest. S2 provides the reading

on the DPM as long as we need to note its value. In the case of pulsed lamp operation, S1

& S2 will be opened on the flat response portion of the boosted lamp profile just before

the transient generated by an accelerator pulse.

This circuit is attractive to record the transient signals upto the time scale of 100

µs/div. At longer time scale, a larger capacitor is required and other aspects of the

circuits must be improved. Also, any capacitor when connected with the electronic

circuits, due the leakage current in the amplifiers and other components, does loose the

charges, which leads to droop problem at longer time scale. To overcome this problem a

digital circuit based on an ADC followed by DAC (Ref.3) is built as they offer the

possibility of holding the I0 value indefinitely. The block diagram of our new back-off

circuit is shown in Fig.8. This back-off circuit is similar to one, which is used in Univ. of

Leipzig & Hahn-Meitner-Institut, Germany. This circuit consists of the back-off loop

(BOL), digital sample and hold circuit (DSH) and control circuitry (CC). The BOL

delivers the feedback current IF of the same magnitude and opposite in polarity as I0 in

order to compensate the initial current I0. When there is no signal at the trigger input of

CC, the analog switch (AD7512) is in position 1, which connects the output of the high

gain non-inverting amplifier A1 to the inverting buffer amplifier A2 and the circuit is in

the sampling mode. In this condition, any signal at the external load resistor (RL) is

nullified by BOL. On receiving a trigger pulse from the sequential delay generator prior

to the arrival of the electron beam, the control circuitry delivers a START pulse to the

analog to digital converter (ADC) of DSH. Using this pulse, ADC resets and the sample-

and-hold amplifier of ADC holds the output voltage of A1 and digitization process starts.

During the conversion, the ADC sends a SJS “high” to the control circuitry. At the end of

conversion, this STS signal goes from high to low. During this transition, the CC

generates two signals: START for Digital to Analog Converter (DAC) and CONTROL

for analog switch. This signal is measured as a stored value of the compensated current

I0, after being buffered by the operational amplifier, at the digital panel meter. The ADC

(1674N) is a 12 bit fast successive approximation converter with in-built sample and hold

amplifier, 10 µs and the DAC (767AD) is a 12-bit converter. The control pulse turns the

analog switch into position 2. Now the back-off circuit is in hold mode and gives IF of the

same amplitude and opposite polarity of I0 and thus I0 is nullified. During this period the

accelerator delivers an electron beam to the sample cell and the transient signal is

captured on the digital storage oscilloscope with the highest sensitivity. The duration of

the CONTROL pulse determines the period of the hold mode. If a pulse with the duration

more than 1 ms is applied at the gate input of the control circuitry, when the circuit is in

hold mode, the back-off circuitry remains in the hold mode as long as the gate signal is

present.

II.4. OPTICAL CABLE ASSEMBLY:

The fibre optic transmission has increased bandwidth resulting in the lower

transmission losses in comparison to the coaxial cable at high frequencies. In either

coaxial or parallel wire, the bandwidth varies inversely as the square of the length, while

in fibre optics cable it varies inversely as the length only. Hence a polymer fibre cable

alongwith the transmitter and Schmitt receiver circuits as shown in Fig.9, procured from

M/s RS Components, is assembled and installed in place of the coaxial shielded cable

between the sequential delay generator and the booster circuit of the xenon arc lamp to

suppress the RF interference from the accelerator cave room. The advantage of using the

optical cable assembly is that the signals are transmitted in the form of photons (light),

which have no electrical charge and, therefore, cannot be affected by the electromagnetic

field as experienced in high voltage environments. Similarly, high magnetic fields from

motors, machineries, transformers etc. have no effect on the optical transmission.

Transmitter:

The transmitter is a high radiance GaAsP light emitting diode optimized

specifically for use with 1 mm core diameter polymer fibre cables. The technical

specifications of the transmitter are as follows:

Forward current : 50 mA max

Reverse voltage : 5 V

Forward voltage (Vf) : 1.9 V at If = 50 mA

Power output : 10 µW at 50 mA seen at the end of 10 meter

length of 1 mm cable.

Peak spectral response : 665 nm

Spectral bandwidth, λ : 22 nm at If = 50 mA

Operating temperature : 0o C to 70o C

Response time : 100 ns

Polymer Fibre Cable:

Polymer fibre cable is made of high performance polymer fibre in single or twin

core configuration and is best suited for visible wavelengths. The technical specifications

of the cable are as follows:

Core outside diameter : 1 mm.

Attenuation : 200 dB/km 665 nm

1500 dB/km 820 nm

Bandwidth : 400 MHz km 850 nm

Operating temperature : -35o C to +85 o C

Numerical aperture : 0.47

Schmitt Receiver:

Schmitt receiver is a sophisticated integrated chip, which is the combination of

photodiode, preamplifier, Schmitt trigger, output device and a voltage regulator. The DC

to 200 Kbits/s operation makes this device ideal for many low-speed data applications.

The technical specifications are as follows:

Peak spectral response : 800 nm

Supply voltage : 4.5 V to 16 V

Supply current : 12 mA max.

Continuous output sink current : 18 mA

Input sensitivity : 3 µW @ λp = 800 nm

Rise time, tr : 150 ns max @ Vcc = 5 V

Fall time, tf : 15 ns

III. RESULTS:

The comparison table shown below gives clear details of our improved pulse

radiolysis measurement system.

Parameter Existing Set-up Improved Set-up

Spectral Range 200-700 nm 200-1600 nm

Long duration measurement 1 ms max. 10 s with PMT

100 s with PD

Sensitivity 20 mV 5 mV

The performance of the photodiode system was qualitatively verified and

compared with the performance of PMT system, where both PMT & PD have good

spectral response. i.e., at 500 & 700 nm using KSCN and electron dosimetry and signals

measured are shown in Fig.10. Using 1 x 10-2 mol dm-3 aerated aqueous solution of

KSCN, PMT gives an absolute value of 0.0329 ± 0.0010 corresponding to a dose of 13.8

Gy (GЄ500 = 21520 dm3 mol–1 cm-1). Under similar conditions, PD gives the absolute

value of 0.0327 ± 0.0010. The absorbance value at 700 nm for the electron dosimetry was

determined to be 0.0894 ± 0.0008. With PD, the absorbance value at 700 nm was 0.0888

± 0.0017. With PMT, different DC values were obtained by changing the PMT voltage

while with PD, the slit width was varied to obtain different DC values.

The performance of both the silicon & germanium photodiodes with the

preamplifier has been tested with nanopure water at 900 nm. Captured signals are shown

respectively in Fig.11. Both the photodiode systems have the rise time of the order of 150

ns.

The baseline signals captured with original bleeder and back-off circuit are shown

in Fig.12.a. In Fig. 12.b, the droop in the base line is clearly seen. This droop problem is

overcome by modifying the PMT Bleeder circuit and installation of new digital back-off

circuit. With these devices, the measurement of steady base line have been taken at

different time scale upto 10 s with PMT and upto100 s with PD. Signals are shown in

Fig.13.

The absorption signal of nanopure water taken with a coaxial cable between the

accelerator cave and the optical detection room is shown in Fig.14a. The signal measured,

under similar condition, after the installation of fibre optic cable is shown in Fig.14.b,

shows the extent of improvement made and ability to measure the signal below 20 mV

which was not possible in the earlier system. A vastly improved KSCN dosimetry signal

measured on a very sensitive scale (3mV signal) with the new device is shown in Fig. 10

c&d.

IV. ACKNOWLEDGEMENTS:

The authors are grateful to Dr. J. P. Mittal, Director, Chemistry and Isotope Group

for his constant encouragement throughout this work. Also, the authors are thankful to

Dr. E. Janata, Hahn-Meitner-Institut , Berlin, Germany and Dr.B. Vojnovic, Gray

Laboratory,UK for their technical guidance while visiting B.A.R.C.

V. REFERENCES :

1. T. Mukherjee in “Atomic, Molecular and Cluster Physics”, ed. S.A. Ahmed, Narosa, New Delhi, 1997, p-299.

2. “Improvements in detection system for pulse radiolysis facility for detection in

near IR region & measurements of weak and long lived signals” V.N.Rao, R. Manimaran, M. Toley, S.J.Shinde, K.G. Girija,R.K. Mishra, S.A.Nadkarni, Hari Mohan, A.V. Sapre and T. Mukherjee, National Symposium on Radiation & Photochemistry, Roorkee, Feb,2001.

3. E. Janata, Rev. Sci. Instrum. 57, 273-275, 1986.

L

INAC

MONOCHROMATOR200-3000nm

V TO ICONVERTOR

D.C.BACK -OFFCIRCUIT

D.S.O.

SAMPLECELL

I.R.PHOTODIODE

PHOTOMULTIPLIER TUBE

Improvements made

P.C.

D.S.O.

MONOCHROMATOR

200-700nm

XENON ARC LAMP&

POWER SUPPLY

SEQ. DELAYGENERATOR

Fibre optic cable

Fig. 1 Block diagram of Pulse Radiolysis setup

ELECTRON PULSE

1 2 3

80 msec.

3 msec.

100 µsec.

0-100 µsec. (ADJ.)

DELAY= 5 msec.

20 msec. 20 msec SYNCH. PULSES

SHUTTER

BOOSTER

DELAY= 19 msec.OSCILLOSCOPE

SIGNAL

Fig.2 Time sychronisation of events

800 1000 1200 1400 1600 18000.1

1

λ nm

Fig.5 Ge.photodiode(Gep800) responsivity

RE

SP

ON

SIV

ITY

(A

/W)

200 400 600 800 1000 1200

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

SE

NS

ITIV

ITY

(m

A/m

W)

λ nm

Fig. 4 Silicon photodiode(PDA55) responsivity

R3

-

+

R1

V input

+Vcc

R5

Vin R4

-Vcc

R2

I output

For, R1=R2 and R3=R4+R5

Iout = R3 x Vin/(R1xR5)

Fig. 6 : Voltage to current converter

S1

C1

A2 S2A3

TO DPM

A

1K

PMTIN

A1

15 K 1 K

0.1µF1K 330 Ω

100 Ω

GATEPULSE

C20.15 µF

20 K

50 Ω

1 K

Fig.7 Analog D.C. back off circuit

SS-- AA22

AA11

RRFF

SIGNALSIGNALIN

11

22

ADCADC DACDAC

CONTROLCONTROLCIRCUITCIRCUIT

A3A3

TRIGGERTRIGGER

CONTROLCONTROL

IIOO

GATEGATE

STARTSTART

SJSSJS

LOADLOAD

RRDD

RRL

IN

SIGNALSIGNALOUTOUT

Fig.8: Digital BackFig.8: Digital Back--off circuitoff circuit

BOLBOL DSHDSH

DRIVE AMP.

SDG

PHOTODIODE

Xe -LAMPBOOSTER

CONNECTOR

RECEIVERAMPLIFIER

LED

F.O.C.

Fig. 9: Optical cable assembly

5 10 15 20 25 30 35 40 45 50-10

0

10

20

30

40PDA55

50 ns pulseλ = 500 nm

D.C. = 254 mV

SIGN

AL (m

V)

TIME (µsec.)

5 10 15 20 25 30 35 40 45 50

-1

0

1

2

3

4

5 PDA5550 ns pulseλ = 500 nmD.C. = 238 mV

SIGN

AL (m

V)

TIME ( µsec.)

5 10 15 20 25 30 35 40 45 50

-5

0

5

10

15

20

25PMT R95550 ns pulseλ = 500nmD.C. = 240 mV

SIG

NAL

(mV)

TIME (µsec.)

5 10 15 20 25 30 35 40 45 50-2

-1

0

1

2

3

4

5

PMT R95550 ns pulseλ = 500nmD.C. = 240 mV

SIG

NAL

(mV)

TIME (µsec.)

(a) (b)

(c) (d)

Fig.10 : Comparison of photodiode & PMT response

KSCN DOSIMETRY

0 1 2 3 4 5-10

0

10

20

30

40

50

60

λ= 900nm

mV

µsecFig. 11a Signal with PDA55 using nanopure water

0 1 2 3 4 5-10

0

10

20

30

40

50

60λ = 900nm

mV

µsecFig. 11b Signal with Gep800 using nanopure water

0 1 2 3 4 5 6 7 8 9 10-10

0

10

20

30

40

TIME (msec.)

λ = 500 nm

SIG

NAL

(mV)

0 50 100 150 200 250 300 350 400 450 500

-40

-30

-20

-10

0

10

λ = 500 nm

SIG

NAL

(mV)

TIME (msec.)

Fig. 12a

Fig. 12b

Fig 12: Baseline with original bleeder &back-off circuit showing the droop

0 1 2 3 4 5 6 7 8 9 10

0

10

20

30

40

50

λ = 900nm

Sig

nal (

mV

)

TIME (sec)

Baseline with PD system &Digital back-off circuit

0 200 400 600 800 1000

0

20

40

60

80

λ = 500 nm

SIGNA

L (mV

)

TIME (msec.)

0 1 2 3 4 5 6 7 8 9 10-10

0

10

20

30

40

50

λ=500nmSign

al (m

V)

Time(sec)

Baseline with PMT modified bleeder & digital dc back-off circuit

0 10 20 30 40 50 60 70 80 90 100-10

-5

0

5

10

15

20

25

λ=500nm

Sign

al (m

V)

TIME Sec

Fig. 13 : Base line of PMT & PD system without droop at longer time scale

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

-20

-10

0

10

20

30

40

50

λ=900nme- BEAM= 50 nsec.

SIGN

AL (m

V)

TIME µsec0.0 0.5 1.0 1.5 2.0 2.5

-10

0

10

20

30

40

50

λ=900nmRISE TIME=150 nsec.e- BEAM WIDTH= 50 nsec.

Sign

al (m

V)

TIME µsec

Fig. 14a: Nanopure water signal with co-axial shielded cable

Fig. 14b: nanopure water signal with optical cable assembly

Fig. 14: Performance of optical cable assembly