An ordinary single-wire proportional counter is used to detect the type and the energy of the radiation passing through the counter. Initial ion pairs in a proportional counter get multiplied by the process of secondary ionization in a region very near to the anode wire where the electric eld intensity is high. The positive ion sheath receding from the anode induces a charge ∆Q on the anode wire and a pulse of voltage ∆V = ∆Q
C is formed at the anode. In an ordinary proportional counter, anode pulse is taken from one end of the anode wire and the anode wire has a very low electric resistance.
A position sensitive single wire proportional counter differs from the normal proportional counter in two respects: (1) pulses can be drawn from both ends of the anode wire and (2) the anode wire is a resistive wire, that is it has a nite resistance per unit length. Now, suppose that the initial ion pairs by a radiation are produced at some point P, very near to the anode inside the detector as shown in gure 7.34. The point P divides the anode wire of total length into two parts: of length X, from P to A and of length (−X) from P to B. The charge ∆Q collected at point P of the anode wire will nd two paths to ow, one from P to A and the other from P to B.
Let ∆Qa and ∆Qb be the parts of the total charge ∆Q (=∆Qa + ∆Qb) that ow through the two paths. As the amount of the charge ow is inversely proportional to the resistance of the path,
∆Qa∝X1
and ∆Q
b∝ X
(
−1)
Therefore,
∆
∆ Q Q
X X
a b
=
(
−)
Or X QQ Q
=
(
a+b b)
The electronic pulses recorded at A and B will have their heights VA and VB proportional, respec- tively, to ∆Qa and ∆Qb. Thus, the position of the initial ionization (point P at distance X from end A)
Figure 7.34 Method of charge division in position sensitive single wire proportional counter
P
A B
X
VB VA
Anode wire
(l−x)
l
may be determined from the ratio V V V
B A+ B
( )
and the total energy of the radiation (which is proportional to ∆Q) may be determined from the sum of the pulse heights (VA + VB).
Multiple-wire proportional counters are used to determine the position and the energy of the ionizing radiations. In their simplest form, an MWPC consists of a num- ber of thin parallel anode wires separated from each other by small distances (≈few mm) and sandwiched between two cathode planes. Cathode planes may be at conduct- ing plates or metallic wires like anodes. The separating distance between anodes and cathode is only few mm so that a small DC potential of about 1 kV produces strong elec- tric eld around anodes, which decreases
rapidly as distance from anode. The lines of electric eld around anode are shown in gure 7.35.
The system is enclosed in an envelope with a thin mylar window to allow the passage of radiations. The airtight outer envelope has ports for feeding counter gas and the high voltage.
A noble gas such as argon mixed with some quenching gas is lled in the counter at atmos- pheric pressure or slightly less than that. As charged particles or ionizing radiations enter the counter they initiate initial ionization. The electrons produced in initial ionization process move towards the anode nearest to them. Very near to the anode, in the region of high electric eld, the electrons produce secondary ionization giving rise to gas multiplication. The positive ion sheath encircling anode moves out towards the cathode. However, the positive ion sheath reced- ing towards cathode induces negative charge on anodes. The quantity of the induced charge is largest on the anode nearest to the point where initial ionization took place. A negative pulse of potential ∆V = ∆Q
C , where ∆QandC are respectively the magnitudes of the induced charge and the capacitance of the detector. Pulses formed at the anode carry information both about the energy of the radiation, which is proportional to the pulse height and the location of the initial ionization that is near to the anode, which has the biggest pulse. To have such a system it is necessary to have individual pulse recording systems (a pre-amplier, an amplier, MCA etc.) associated with each wire. This is both costly and unmanageable if the number of wires is large.
The alternate method is to use the method of charge division. In this method, one end of alternate anode wires is connected to each other and thus there are two outputs one from each side of the anode wires. The difference in the size of the signal potential from the two set of outputs gives information about the position of the event while the sum of the pulse heights from the two out- puts give the energy of the radiation.
MWPCs are operated under gas ow mode and care is to be taken that the applied potential does not drive the system to the GM or to the region of limited proportionality.
Anode wires
Strong electric field around anodes Figure 7.35 Electric eld around anodes in a
multiwire proportional counter
Figure 7.36 A multiwire proportional counter setup Window for radiations
Cathodes
Anodes
GND GND
GND HT
supply
HT supply
Pre-amplifiers
7.8 DCN NRN
Neutrons being neutral do not interact with atomic electrons of the absorbing medium. Neutron interaction with electron through the magnetic moment is negligible. As such, detection of neu- tron is not direct. Neutrons scatter nuclei that recoil with sufcient energy if they are light. The maximum energy Emax that may be given by a neutron of kinetic energy E to a nucleus of atomic mass number A is
E A E
max = A +
( )
4 12
Light nuclei such as protons (nucleus of hydrogen atom) recoil with large energies if scattered by a fast neutron. Detecting of the recoiling nucleus is one way of detecting neutrons. Charged recoiling nuclei can be easily detected either by a gas-lled counter or a scintillation spectrom- eter. As the energy distribution of recoiling nucleus is intimately related to the energy of the neutron, with proper bookkeeping it is possible to determine the energy of the incident neutron.
The other possible method of detecting neutron is through nuclear reactions initiated by neutron.
For example, thermal neutrons have large cross-sections (probability) for the following reactions:
5 10
0 1
2 4
3
7 2 3
B+ n = He+ Li*+ . MeV
3 7
3 7
0
0 0 48
Li*= Li+ g + . MeV and
92 235
0
1 2 3
U+ =n Two fission fragments+ or fast neutronss+200MeV
In the rst reaction, a boron nucleus on absorption of a thermal neutron emits an a particle and leaves the excited nucleus 37Li*, which decay to the ground state by emitting g rays. The emitted
a particle can be easily detected either by a gas-lled counter operated in the ionization region, proportional region or GM region. Gas-lled counters having boron triuoride gas as counter gas are often used to detect the ux of thermal neutrons. Fast neutrons may also be detected if they are converted into thermal neutrons by passing them through a block of parafn wax. Parafn wax is a very good neutron moderator (material that quickly reduce the energy of neutrons with- out much loss in their numbers.) as it contains light nuclei (hydrocarbon). Alternately, a thin coating of boron on the cathode of a normal gas counter makes it a thermal neutron detector.
In the second reaction, thermal neutrons has a large probability to initiate ssion in 235U that results in the production of two heavy ssion fragments each having high charge. Fission fragments being highly ionizing can be easily detected in an ionization chamber. Ionization chambers with a thin coating of enriched 235U on cathode (called ssion chambers) serve as good thermal neutron detector.
Counters lled with CH4 or 4He gas are sensitive to high-energy neutrons as the scattered pro- tons or a particles may be recorded. If the counter is operated in proportional region information about the energy of the incident neutron may also be obtained.
Charged particles produced as a result of neutron scattering or in nuclear reactions initiated by neutrons, may also be detected by a scintillation detector. Lithium-loaded glass scintillators are used for neutron detection. Plastic scintillators, which have high content of hydrogenous material, may themselves serve as scatterer. Protons emitted as a result of neutron scattering are registered by the detector. Neutron detection, particularly with scintillation detectors, the pres- ence of large g ray background often creates problem. Neutron interaction with any material, in general, produces almost 10 times more ux of g rays than charged particles. However, proper shielding and designing of the experiment may solve the problem. For example, g rays of about 1 MeV emitted in nuclear ssion are absorbed up to 90% by a 5 cm thick lead shielding, while only 0.1% ssion neutrons are absorbed by the same thickness of lead.
The shape of the electronic pulse produced in any detector depends on how the energy is deposited in the detector by the radiation. As different types of particles deposit energy by dif- ferent processes, particularly the decay time of the pulse is quite different for different particles.
Electronic analysis of detector pulses may be used to sort out the pulses produced by desired radiation. This is called pulse-shape discrimination and is used to remove g background in neu- tron detection by plastic scintillators.
Determination of the kinetic energy of neutrons is more involved than that of other radiations.
A very common method is to use the time of ight technique. In this method, as the name suggests, the time that neutron takes in covering a known distance is measured which gives the velocity of the neutron. The kinetic energy may be determined from the speed and the mass of the neutron. In actual experiment, a thin detector (say a plastic detector) and a total absorbing thick detector (liq- uid scintillator) are kept a few meters apart in the path of the neutron. The time difference between the neutron pulse in thin detector and the thick detector is very accurately determined. Fast elec- tronic circuits can very accurately determine time differences of the order of 10-11 seconds or less.
Knowing the distance between the detectors and the time-of-ight of neutron the energy of neutron may be determined. A check on the energy value determined from the time of ight may be made from the thick detector pulse that has correlation with neutron energy. Time of ight method may also be used for determining the energy distribution of neutrons. The distribution of ight time is a replica of the energy distribution. In actual experiments, ight time is converted into electronic pulses and from the distribution of pulse heights neutron energy distribution may be found.
7.9 DCN NRN
Neutrino’s feeble interaction with matter makes them uniquely valuable as astronomical mes- senger. Neutrinos move out undiverted through a huge mass of matter and do not suffer any deection in interstellar magnetic eld. Most of the neutrinos oating around were born some 15 billion years back soon after the birth of universe. New neutrinos are constantly generated at nuclear power stations, accelerator centres, explosion of supernova, in collision, and death of stars. There are three avours of neutrinos. Neutrinos may interact with matter via neutral current (Z-boson) or through charged current (W-boson). In neutral current interaction, neutrino loses some energy and momentum but no information about the avour is left. In charged current interaction, the neutrino is transformed into its partner lepton. Experiments based on following methods are being carried out at international level to detect and study the properties of neutrino.