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.
diameter holds 50,000 tons of ultrapure water. Two arrays of photomultiplier tubes consisting of, respectively, 11,146 and 1885 tubes surround the tank and look for Cerenkov rings. The observa- tory tracked neutrinos emitted from the supernova 1987a.
7.9.2.2 Irvine–michigan–brookhaven (IMB)
This is a joint project of Irvine, Michigan, and Brookhaven universities to track neutrinos. The laboratory has been set at Marton Salt Company’s Fairport mine at the shore of Lake Erie, USA.
A cubical tank of 17 × 17.5 × 23 m holds 2.5 million gallons of ultrapure water. The tank is sur- rounded by 2048 photomultiplier tubes (gure 7.37).
Figure 7.37 The Cerenkov radiation cones for muon- and electron-neutrino induced events
νe
Cerenkov radiation cone
νµ µ
Muon neutrino
Muon
Electron
neutrino Electron shower
The cerenkov radiation from a muon produced by a muon neutrino event yields a well defined circular ring in the photomultiplier detector bank.
The cerenkov radiation from the electron shower Produced by an electron neutrino event produces multiple cones and therefore a diffuse ring in the detector array.
7.9.2.3 Antarctica muon and neutrino detector arra (AMANDA)
It is a neutrino telescope located beneath the Amundsen-Scott South Pole Station. In 2005, after 9 years of operation, AMANDA ofcially became part of its successor project, the IceCube Neutrino Observatory.
AMANDA consists of optical modules, each containing one photomultiplier tube, sunk in Antarctic ice cap at a depth of about 1500–1900 m. In its latest development stage, known as AMANDA-II, AMANDA is made up of an array of 677 optical modules mounted on 19 separate strings that are spread out in a rough circle with a diameter of 200 m. Each string has several dozen modules, and was put in place by ‘drilling’ a hole in the ice using a hot-water hose, sinking the cable with attached optical modules in, and then letting the ice freeze around it. AMANDA detects very high energy neutrinos (50+ GeV), which pass through the earth from the northern hemisphere and then react just as they are leaving upwards through the Antarctic ice. The neutrino
collides with nuclei of oxygen or hydrogen atoms contained in the surrounding water ice, pro- ducing a muon and a hadronic shower. The optical modules detect the Cherenkov radiation from these latter particles, and by analysis of the timing of photon hits one can approximately deter- mine the direction of the original neutrino with a spatial resolution of approximately 2°.
7.9.2.4 Sudbury neutrino observatory (SNO)
It is situated 2 km underground in Vale Inco’s Creighton Mine in Sudbury, Ontario, Canada uses heavy water. Neutrino may break up deuterons in heavy water producing neutrons, which may be re-absorbed giving burst of g rays.
7.9.2.5 MiniBooNE
It is an experiment of Fermi Lab, USA. A beam of muon neutrinos is directed at a tank lled with mineral oil which has scintillation properties. The scintillator tank is viewed by 1280 PMTs. The detector will be able to detect low-energy neutrinos that do not produce Cerenkov radiations.
7.9.2.6 MINOS (Main Injector Neutrino Oscillation search)
It is an experiment to study neutrino oscillations. Two detector setups, the near one at the Fermilab and another 735 km away at Minnesota, detected neutrinos to look for their oscillations.
Neutrino oscillation is a quantum mechanical phenomenon in which a neutrino of a specic
avour is detected with a different avour as it moves. These avour oscillations occur periodi- cally. Neutrino oscillation may lead to the nite mass of the neutrino and may result in the fall of standard model.
7.9.3 ndian nitiative for Neutrino tudies
A laboratory named ‘Indian Neutrino Observatory (INO)’, to study the properties and interactions of neutrinos, is going to be developed
at the Bodi-West hills on the coast of southern Tamil Nadu state. The envi- ronment and forest ministries have given clearance to the project. Thick cover of rock (about 1000 m high) will provide the natural lter for removing all other radiations except the neutrinos. Moreover, a very large electromagnet will further remove the charged particle background. It is expected that the laboratory will become a leading international facil- ity for the study of neutrinos. Facility costing $270 m will be the fth such
facility in the world (gure 7.38). Figure 7.38 Proposed site for the Indian Neutrino Observatory
7.10 D-A NCAR RAC DCR (ND)
Heavy charged particles passing through insulating solids including crystals, inorganic glasses, and plastics produce sub-microscopic tracks that may be seen by a high-resolution electron microscope. The linear region of radiation-damaged material is called the track.
Though the initial tracks are of sub-microscopic size, their size may be enlarged, xed, and stabilized by chemical treatment, called etching, with a suitable chemical such as NaOH, hydro-
uoric acid, and strong alkalis depending on the material of the detector. The particle tracks after etching may be observed by an ordinary optical microscope.
7.10.1 echanism of rack ormation Heavy charged particle passing through an insulating material ionizes the atoms along its trajectory forming a cylindrical region
lled with positively charged ions, which violently repeal each other. This almost cylindrical space along the trajectory of the particle has strain locked in the region.
Such regions because of their higher dif- fraction contrast and chemical reactivity may be seen by a high-resolution electron microscope. Figure 7.39 shows the photo- graph of a track of ssion fragment in an SSNTD.
Different insulating materials are sen- sitive to different types of radiations. For example, organic polymers are good mate- rial for nuclear particles such as a s and other heavier ions. Some organic polymers even show tracks of deuterons and protons.
The selectivity of the detecting material is due to the ‘minimum threshold damage density’, which has a well-dened value for each material and below which no tracks may be formed.
A big advantage of SSNTD, particularly over nuclear emulsions, is its capability of
detecting heavy and highly charged particles such as ssion fragments, with great efciency in presence of large background of g rays and neutrons. However, no timing information about the recorded event may be obtained from SSNTD.
Because of its relatively low cost, ease of handling, permanent record, non-requirement of intricate electronics, and so on, SSNTD are nding applications in nuclear physics, geochronol- ogy, cosmology, biology, bird altimetry, seismology, elemental analysis, lithography, material science, and so on.
Figure 7.39 Photograph of a ssion fragment track in a SSNTD
7.11 CN A DCR
The choice of a detector depends on the type of the radiation to be detected, expected energy of the radiation, the intensity (the expected number of radiations per unit time) and the environ- ment where the experiment is to be performed. Broadly speaking, there may be two types of experimental set-ups; the rst where experiments are done off-beam and the second in-beam experiments. In off-beam experiments, there is no beam of accelerated particles and, therefore, the unwanted radiation background is not very much. Normal background due to cosmic rays and natural radioactivity can be shielded using lead shields of appropriate thickness. As such, in off-beam experiments there is not much problem regarding the breakdown of the detector under high radiation background. However, in the case of those experiments that require measurements to be done in-beam, when the beam of accelerated particles is hitting the target, special detectors that do not get damaged by the high background radiations need to be used.
Some experiments require only to detect the presence of the radiations and to count their number in a given time. GM counter is often used for this purpose. Although a GM counter could not distinguish between different radiations but it has almost 100% detection efciency, once the radiation has reached the active volume of the counter. End-window GM counter further facili- tates the entry of low-energy radiations in the active volume. Moreover, GM counter is rugged, stable, and requires very little electronic modules for operation.
In case the type and the energy of the radiation are to be determined, detectors such as pro- portional counter, scintillation detectors, and solid-state detectors may be used. In such cases, the size or thickness of the detector becomes very important. As is obvious, the thickness of the detector must be more than the range of the radiation in the detector material so that the radiation is completely stopped in the detector and deposit all its energy in the detector. A detector of larger size has higher detection efciency, but may not be of advantage if radiations of low energy are to be recorded. For example, a 4″ × 4″ × 4″ NaI(Tl) crystal is very good for the recording of 100 keV to about 1.5 MeV g rays, but if it is required to record only 100 keV g s then a detector of smaller size, say 1″ × 1″ × 1″ is good. It is because a smaller detector will have very small detec- tion efciency for high energy g s and large detection efciency for lower energy g s. As a result, the low energy g s will be efciently detected and recorded by a small crystal and at the same time the background activity due to the higher energy g s present in the environment will be consider- ably reduced due the low detection efciency of the detector for high-energy g rays. It is for of this reason that thin detectors are used for the detection and recording of X-rays, b , and other charged particles.
Expected intensity (count rate) of the radiations also plays an important role in selecting the detector. If the expected intensity is high, a detector that produces fast pulses of low-pulse dura- tion is to be used. This will avoid pile-up of pulses. Fast electronic modules that may handle high count rates are required to be used in such cases.
From the foregoing discussion, it is clear that proper planning of the experiment is very essen- tial. As a matter of fact most of the time computer simulation of the experiment is done before actually doing the experiment to check the suitability of the experimental set-up.
Exercise p-7.16: Describe the working of a gas-lled proportional counter, explaining the pro- cess of gas multiplication and quenching. Why are cylindrical counters preferred over parallel plate type?
Exercise p-7.17: Discuss the working of a GM counter explaining the pulse formation, dead time, and recovery time of the counter. On what factors does the pulse size of a GM counter depend?
Exercise p-7.18: Describe the process of scintillation and discuss the characteristics of a good scintillator.
Exercise p-7.19: With the help of a block diagram explain the working of a NaI(Tl) scintillation spectrometer. What is the purpose of thallium activation of the crystal?
Exercise p-7.20: Draw a rough sketch of the spectrum of 662 keV g rays of 137Cs source taken with a NaI(Tl) detector and explain the origin of each peak of the spectrum.
Exercise p-7.21: In what respect Ge(Li) detector is different from HPGe detector? Which is better and why?
Exercise p-7.22: Solid-state detectors are better than any other detectors for g ray detection. Do you agree with the statement, if yes, give reasons of your answer?
Exercise p-7.23: What is meant by the resolution of a detector setup? On what factors the reso- lution of a system depends? Why the resolution of semi-conductor detectors is so good as com- pared to other detectors.
Exercise p-7.24: Write notes on: (i) Position sensitive counters, (ii) Pulse-shape discrimination and phoswich detectors, (iii) BGO as a detector for high energy g rays.
Exercise p-7.25: How will you detect and determine the energy of a fast neutron?
Exercise p-7.26: Review the major international efforts on foot to detect neutrinos.
Exercise p-7.27: Describe in detail the working principle of SSNTD and some of its applications.
Exercise p-7.28: Derive an expression for the maximum energy that can be lost by a particle in an interaction with another particle.
Multiple choice questions
Note: It is possible that more than one alternative is correct in case of some questions. In such cases, all correct alternatives should be selected.
Exercise M-7.1: Which of the following radiations show exponential absorption?
(a) Mono-energetic a particles (b) mono-energetic b particles
(c) g rays (d) mono-energetic neutrons
Exercise M-7.2: Cross-section for photoelectric effect in an atom is maximum for (a) any bound electron (b) valance electron
(c) K-shell electron (d) M-shell electron Exercise M-7.3: Auger electrons are emitted when
(a) a f ray is converted into an electron
(b) excitation energy of an atom is given directly to the atomic electron (c) a rays knockout an electron from the atom
(d) in f decay internal conversion takes place.
Exercise M-7.4: The mass absorption coefcient of a certain g ray in aluminium (density = 2700.0 kg/m3) is 0.5 cm2/g. The linear absorption coefcient of the g ray is
(a) 0.5 m-1 (b) 1.35 m-1 (c) 0.185 cm-1 (d) 1.35 cm-1
Exercise M-7.5: Maximum energy (in MeV) that a 0.511 MeV g ray can transfer to a free electron is
(a) 0.511 (b) 0.255 (c) 0.170 (d) 0.128
Exercise M-7.6: The Compton wavelength (in meter) of an electron is of the order of
(a) 10-15 (b) 10-12 (c) 10-9 (d) 10-6
Exercise M-7.7: A heavy charged particle, such as proton, may also undergo collisions with the nuclei of the absorbing material just like the collisions it suffers with the atomic electrons of the absorbing medium, but we do not take energy loss by nuclear collisions in consideration while calculating the stopping power. Why?
(a) Nucleus is much smaller in size than the proton
(b) The charge on the nucleus is much larger than the electronic charge (c) Compton wavelength is smaller than the nuclear dimensions
(d) Energy loss is inversely proportional to the mass of the target particle and nuclear mass is much larger than the mass of the electron; hence, negligible energy is lost in colli- sions with nuclei.
Exercise M-7.8: A proton and a deuteron of same velocity are moving in aluminium absorber, the ratio of the stopping power of proton to deuteron is
(a) 1 (b) 2 (c) 4 (d) 0.5
Exercise M-7.9: Approximate range (in meter) for a 5 MeV a particle in air is
(a) 0.05 (b) 1.0 (c) 0.5 (d) 0.005
Exercise M-7.10: The radiative energy loss is
(a) directly proportional to the kinetic energy of the particle (b) inversely proportion to the kinetic energy of the particle (c) directly proportional to the mass of the particle
(d) inversely proportional to the square of the mass of the particle.
Exercise M-7.11: Cerenkov radiations are emitted
(a) when a light charged particle moves in a medium with a velocity larger than the velocity of light in the medium
(b) when bmfor the particle<1 (c) when bmfor the particle>1
(d) when the Cerenkov radiations are emitted in a cone of semi-angle q = b m Exercise M-7.12: A GM counter detects g rays primarily through
(a) Compton scattering with the counter gas (b) photoelectric effect with counter gas
(c) photoelectric effect with the cathode of the counter (d) pair production with the counter gas