detection system for near ir region, weak and long …
TRANSCRIPT
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