nuclear electronics for radiation measurements · 2019. 1. 1. · nuclear electronics for radiation...
TRANSCRIPT
Nuclear Electronics for
Radiation Measurements
Dr. BC Choudhary, Professor
Applied Science Department, NITTTR, Sector-26, Chandigarh-160019.
RADIATION DETECTORS
Based upon the effect produced when a charged particle or
radiation passes through the matter
IONIZATION or EXCITATION
Solid State Detector
Semiconductor Detectors
Gas Ionization Detectors
Ionization Chambers
Proportional Counters
Geiger-Mueller (GM)Counters
Scintillation Detectors
General Properties of Radiation Detectors
In any of the detector; Net result of radiation interaction
Appearance of a given amount of charge within detector active medium.
For a single interaction
At time t = 0 to tc,
Total amount of charge generated = Q
Assumption:
Low irradiation rate
each individual interaction
give rise to a current that is
distinguishable.
Modes of Detector Operation
Radiation detectors generally used in Three Modes
Pulse Mode
Current Mode
Mean Square voltage mode
o Pulse mode is easily the most commonly applied, but
current mode also find many applications.
o MSV mode is limited to some specialized applications
that make use of its unique characteristics.
Pulse mode is most frequently used.
Pulse Height Spectrum
The Pulse height spectrum is the distribution
of pulse height and gives the idea of
constitution of flux incident on the detector
or emanated from the source.
Pulse amplitude carries information
regarding charge generated by particular
radiation interaction in the detector
(a) Differential (b) Integral pulse height
spectra for an assumed source of pulses.
(a)
(b)
Represented in two different modes.
• Differential mode
• Integral mode
Number of pulses
between H1 & H2
2
1
H
H
dHdH
dN
Total number of pulses
under distribution N0
0
dHdH
dN
Properties of Detector
Energy Resolution : An important property of a detector
Ability of a given detector or
measurement to resolve fine details of
two events
Good Energy Resolution
Poor energy resolution
Response functions for detectors with
good and poor resolution
FWHM : Width of the distribution at a
level that is just half the maximum height
(ordinate) of the peak
0H
FWHMR Resolution
FWHM
Energy
Energy
Co
un
ts
Potential sources for imperfect
energy resolution of a detector :
Operating Characteristics of detector &
their drift.
Random Noise in detector and associated
electronics.
Statistical fluctuations in number of charge
carriers produced due to interaction of
quantum of radiation of same energy.
2 .
2
.
2
.int
2
noisestnoiserantotal FWHMFWHMFWHMFWHM
Resolution is a dimensionless fraction expressed as a percentage.
100% Energy
FWHMR
Smaller the figure for ‘R’; better the
detector able to distinguish between
two radiations whose energies lie near
each other.
Detection Efficiency
Measure of response of a detector to the incident radiations
In case of photon counters, all the quanta do not deposit their
complete energy and hence are not recorded. The concept of
efficiency becomes important for such detectors.
Efficiency of a detector depends upon:
(i) Detector medium
(ii) Dimensions of detector.
(iii) Source to detector distance.
(iv) Nature of radiation being measured.
Efficiency defined in two different ways
• Absolute Efficiency
• Intrinsic Efficiency
Absolute Efficiency
sourcethebyemittedquantaof.No
recordedpulsesof.Noabs
Depends on detector properties and also on details of counting geometry
Intrinsic Efficiency
ectordetonincidentquantaof.No
recordedpulsesof.Noint
No longer include the solid angle subtended by the detector
For Isotropic Sources; Absolute and intrinsic efficiencies are
related through
absint
4
- solid angle of detector seen from
actual position.
Much more convenient to tabulate values of intrinsic efficiency
because of milder geometric dependence.
Counting Efficiencies
Counting efficiencies also categorized by the nature of the
event recorded
Total Efficiency : If we accept all pulses from the detector . In this case
all interactions, no matter how low in energy, are assumed to be counted.
Entire area under the spectrum is considered.
Photo peak Efficiency: Only those interactions that deposit the full
energy of the incident radiation are counted.
Peak -to-Total ratio : Total
Peakr
A measure of figure of merit
of detectors
Often preferable for an experimental standpoint to use only peak
efficiency because it is not sensitive to perturbation effects.
Energy Resolution
Comparison of pulse height spectra recorded by
three different detectors.
Semiconductor detectors Si(Li)
have best energy resolution
among all types of the radiation
detectors
Efficiency Comparison
Peak efficiency for five different detectors as a function of incident
X- or gamma ray energy.
Measurements using Radiation Detectors
Energy Measurements
• Measures the energy of the incident radiations.
• It is used in counting systems. Mostly single mode
• Records the spectrum from various types of interaction
taken place in the detector in term of voltage pulse.
Time Measurements
• Measure the time elapse between two incident events
• Used in coincidence mode
Radiation application normally involve Energy measurements.
The output signal amplitude is further processed to extract the
desired information.
Energy Measurement
What is the shape of the spectrum for a large detector?
Nuclear Electronic Instruments
Block Type : Each unit is self contained and
independent block.
NIM Standard: Modular form- Units are not self
contained. They can be fitted in a BIN and Power
supply as per requirement.
Generally Categorized in Two Types
Self Contained Block Modules
NIM Electronics
NIM units along with Bin for
power supply
Counting Instrumentation
A Simple Counting System
Proportional Detector Counting System
GM Counting System
Scintillation Counting System
Single Channel Counting System
Dual-channel photon-counting system
Simple Coincidence Counting System
Time Measurements
Typical Fast /slow Timing System for Coincidence
measurements with scintillatores
Pulse Processing and Shaping
Purpose is to extract information from the pulses produced
by the radiation detectors.
Major steps involved to process and shape the pulse to get
required information are :
Device Impedance
Coaxial Cables
Pulse Shaping
Linear and logic pulses
Pulse counting System
Pulse Height Analysis System
Device Impedance
For ideal device response
High Input Impedance
Low Output Impedance
Important to consider is the impedance of the device that
comprise the signal processing chain.
Idealized input and output configuration
LO
LSL
ZZ
ZVV
Coaxial Cables All interconnections of components in a signal chain for nuclear detector
pulses is carried out using shielded coaxial cable
• Shielded construction is designed to
minimize pickup of noise from stray
electric and electromagnetic fields.
• To preserve the flexibility of the cable,
the outer shield is usually made of
braided strands of the copper wire.
In signal cables, the important specifications are the characteristic impedance
and the capacitance per unit length.
In cables intended to carry bias voltage to detectors, the maximum voltage
rating is important.
Velocity of propagation is proportional to
For Polyethylene v 66% of c k
1k - dielectric constant of
conductor
No cable is a perfect transmission line. There will always be dissipative losses due
to imperfect dielectric and resistance of the center conductor that will result in
some attenuation & distortion of the transmitted pulse.
Various Type of Coaxial cables
Various Coaxial
Cables &
Accessories
Adapters & Terminators
Various types
of terminators
Various types
of adapters
Pulse Shaping
Often desirable to change the shape of the signal pulse in
some predetermined fashion.
• Most common application is in
processing a train of pulses
produced by a preamplifier.
• If the rate of interaction in the
detector is not small, these pulses
will overlap one another giving
rise to apparent variation in
amplitude
Amplitude carries the basic information of charge deposited in detector, the
pileup of pulses, can be a serious problem.
Ideal solution is to shape the pulses in such a way as to produce a pulse train.
Here all the long tails have been eliminated, but the amplitude carried by the
maximum amplitude of the pulse has been preserved.
Linear and Logic Pulses
Virtually all radiation detector signal chains start out with
linear pulses and, at some point, a conversion is made to logic
pulses based on some predetermined criteria.
In any pulse processing system, important to distinguish
between two types of signal pulse.
• Linear pulse: A signal pulse that carriers information through the
amplitude, and sometimes by its shape as well. A sequence of linear pulses
may therefore differ widely in size and shape characteristics.
• Logic pulse: A signal of pulse of standard size and shape which carries
information only by its presence or absence or by the precise time of its
appearance.
NIM Standard
Nuclear Instrument module (NIM) standard recommends
• Shaped linear pulses be one of the specific dynamic range
0 to + 1V (for ICs)
0 to + 10V (for transistor-based circuits)
• Standard logic pulses are used in normal applications when the
potential counting rate does not exceed 20 MHz.
Output (must deliver) Input (must respond to)
Logic 1 +4 to +12V +3 to +12 V
Logic 0 +1 to -2V +1.5 to -2V.
Components Common to many Applications
Preamplifiers
Fundamental output of all pulse type radiation detectors is a burst of charge
Q that is liberated by the incident radiation.
In most of detectors except G-M tube and scintillation counter, the charge is
so small that it is impractical to deal with the signal pulses without an
intermediate amplification step.
• The first element in a signal processing chain is
therefore often a preamplifier
• Provides an interface between the detector and the
pulse-processing and analysis electronics that
follows
Preamplifier is usually located as close
as possible to the detector.
Preamplifiers in Semiconductor Detectors
Expanded views of detector capsules with
horizontal & vertical dipstick cryostat. In
these detectors, preamplier is also kept at low
temperature.
Semiconductor Detectors
Detector dipped in LN2 cryostat
Detector
Capsules
&
Assemblies
• One function of preamplifier is to terminate the capacitance
quickly and therefore to maximize S/N ratio.
• Also serves as an Impedance matching device.
From S/N point, always preferable to minimize the capacitive
loading on the detector, and therefore long interconnecting
cables between detector and preamplifier should be avoided.
Preamplifier conventionally provides no pulse shaping, and
its output is a linear tail pulse.
Rise time of the output is kept as
short as possible,
Decay time of the pulses is made
quite large for full collection of
charge from detector. Preamplifier pulses
Preamplifier can be either the Voltage Sensitive or
Charge Sensitive type.
(Assume A >>> R2/R1) in
1
2out V
R
RV
Voltage Sensitive : More conventional in many electronic applications
where C is the input capacitance C
QVi
• Consist of a simple configuration that
provides an output pulse whose amplitude
is proportional to the amplitude of the
voltage supplied to its input terminals.
Detector capacitance may change with operating parameters. In
this situation, a voltage sensitive preamplifier is undesirable
because the proportionality between Vmax and Q is lost.
Charge Sensitive Configuration : In this circuit, the output voltage
is proportional to the total integrated charge in the pulse provided to
the input terminals, as long as the duration of the input pulse is short
compared to the time constant RfCf.
Assume A >> (Ci + Cf)/ Cf
fi
inoutC)1A(C
QAAVV
f
outC
QVor
Thus changes in the input capacitance no longer have an
appreciable effect on the output voltage.
Detector Bias & Bin Power Supply
Bin and power supplies for NIM modules
Low and High detector bias supplies
Amplifier
Front & Rear views
Output from the preamplifiers are provided to a linear
amplifier element in the pulse processing chain.
Conventionally provides two primary functions:
Pulse shaping and Amplitude gain.
Accept tail pulses as an input of either polarity, and
produces a shaped linear pulse with standard polarity
and span.
Standard amplifiers are provided with
• Variable, amplification factor or gain,
• Shaping time,
• Pole-zero cancellation and base line restoration.
• Provides output pulses: Unipolar & bipolar
Pulse with pole-Zero adjustment
Pulse without and with pole-Zero adjustment
Output Pulses & Pole-Zero
Type of output pulses
from amplifier
Unipolar Output
Effects of Pole-Zero Cancellation
Pulse Counting System
Integral Discriminator
Differential Discriminator
(Window mode)
Single Channel Analyser
SCAs monitor the height of
the shaped linear pulses and
convert to logic pulses
according to discriminator
mode selected.
• In a counting system, the logic
pulses must be accumulated
and their number recorded
over a fixed period of time
• Devices used for this purpose
include counters, timers or
counting rate meters
Linear to logic pulse conversion
Timer, Counters & Counter-Timer
Multi Channel Analyzer (MCA)
Next to simple counting of pulses,common procedure in
nuclear measurements involves recording the amplitude
distribution of pulses produced by a detector.
An analogy representing pulse-height sorting function
in the multichannel pulse height analyzer
Objective is to deduce
properties of the incident
radiations from the position
of peaks in the recorded
spectrum.
Device designed to carry this
function is known as
multichannel pulse height
analyzer (MCA)
MCA consist of three main sections:
• Analog to digital converter (ADC),
• Memory and
• Display
ADC measures the amplifier pulse peak amplitude and convert it to a digital
number. This number represents the address of a memory location in the
analyzer memory. The number of times a pulse of each height has been detected
is accumulated in the analyzer memory to form the spectrum of pulse heights.
Subsequently, this information can be displayed as a picture of the analyzer
energy spectrum.
MCA can performs a number of functions like data acquisition and storage,
dead time correction, display manipulation, linear and quadratic energy
calibration, spectrum stripping and smoothening etc.
Stored data can be transferred to either on storage media or listed out on
printers.
Analog to Digital Converter (ADC)
Typical Spectrum
Comparison of
spectrum recorded
using a NaI(Tl)
and Germanium
detectors.
Typical X-ray Spectra recorded with a Si(Li) low energy photon
detector.
Conversion electron spectrum from 140Ba decay taken with
Si(Li) detector in an electron spectrometer.
Bismuth Germanate (Bi4Ge3O12)
(BGO)
A HPGe detector surrounded by a
Compton suppression system made up
of NaI(Tl) and BGO scintillators Pulse height spectra of Co60 gamma-
ray source using Anti-Compton shield
Intrinsic Photopeak efficiencies for BGO
and NaI(Tl) scintillators of equal size. Measurements of light pulse shapes
from BGO and NaI(Tl)
Comparison of Response of BGO and
NaI(Tl) Scintillators
Detector arrangements for In-Beam Studies
• Placement of detectors a
appropriate angles is very
important to minimize
angular distribution effects.
Multi-Detector Systems
INGA at BARC-TIFR, Mumbai
GDA at IUAC Delhi
Email: <[email protected]>
Cell: 09417521382