Nuclear_Physics_Lecture_12.pdf

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(1)Synchrotron A synchrotron is a particular type of cyclic particle accelerator, descended from the cyclotron, in which the accelerating particle beam travels around a fixed closed-loop path The magnetic field which bends the particle beam into its closed path increases with time during the accelerating process, being synchronized to the increasing kinetic energy of the particles.

(2) Synchrotron The synchrotron is one of the first accelerator concepts to enable the construction of large-scale facilities, since bending, beam focusing and acceleration can be separated into different components The most powerful modern particle accelerators use versions of the synchrotron design. The largest synchrotron-type accelerator is the 27kilometre-circumference (17 mi) Large Hadron Collider (LHC) near Geneva, Switzerland, built in 2008 by the European Organization for Nuclear Research (CERN).

(3) Synchrotron The synchrotron principle was invented by Vladimir Veksler in 1944 Edwin McMillan constructed the first electron synchrotron in 1945 The first proton synchrotron was designed by Sir Marcus Oliphant and built in 1952 The synchrotron evolved from the cyclotron, the first cyclic particle accelerator. While a classical cyclotron uses both a constant guiding magnetic field and a constant-frequency electromagnetic field (and is working in classical approximation), its successor, the isochronous cyclotron, works by local. variations. of. the. guiding. magnetic. field,. increasing relativistic mass of particles during acceleration. adapting. the.

(4) Synchrotron In a synchrotron, this adaptation is done by variation of the magnetic field strength in time, rather than in space. For particles that are not close to the speed of light, the frequency of the applied electromagnetic field may also change to follow their non-constant circulation time. By increasing these parameters accordingly as the particles gain energy, their circulation path can be held constant as they are accelerated This allows the vacuum chamber for the particles to be a large thin torus, rather than a disk as in previous, compact accelerator designs. Also, the thin profile of the vacuum chamber allowed for a more efficient use of magnetic fields than in a cyclotron, enabling the cost-effective construction of larger synchrotrons.

(5) Synchrotron The maximum energy that a cyclic accelerator can impart is typically limited by the maximum strength of the magnetic fields and the minimum radius (maximum curvature) of the particle path Thus one method for increasing the energy limit is to use superconducting magnets, these not being limited by magnetic saturation Electron/positron accelerators may also be limited by the emission of synchrotron radiation, resulting in a partial loss of the particle beam's kinetic energy. The limiting beam energy is reached when the energy lost to the lateral acceleration required to maintain the beam path in a circle equals the energy added each cycle.

(6) Synchrotron Unlike in a cyclotron, synchrotrons are unable to accelerate particles from zero kinetic energy; one of the obvious reasons for this is that its closed particle path would be cut by a device that emits particles. Thus, schemes were developed to inject pre-accelerated particle beams into a synchrotron. The pre-acceleration can be realized by a chain of other accelerator structures like a linac, a microtron or another synchrotron; all of these in turn need to be fed by a particle source comprising a simple high voltage power supply, typically a Cockcroft-Walton generator.

(7) Synchrotron Starting from an appropriate initial value determined by the injection energy, the field strength of the dipole magnets is then increased. If the high energy particles are emitted at the end of the acceleration procedure, e.g. to a target or to another accelerator, the field strength is again decreased to injection level, starting a new injection cycle. Depending on the method of magnet control used, the time interval for one cycle can vary substantially between different installations.

(8) Synchrotron. https://www.youtube.com/watch?v=yjDkj_LiScM.

(9) Circumference: 6.3 km. Fermilab Accelerator Complex: The Tevatron.

(10) Circumference: 27 km. Site of the LHC at CERN in Geneva.

(11) GM Counter. Detector. An instrument for measuring ionizing radiation used widely in applications such as radiation dosimetry, radiological protection, experimental physics and the nuclear industry It detects ionizing radiation such as alpha particles, beta particles and gamma rays using the ionization effect produced in a Geiger–Müller tube. Geiger – Muler Counter is a gas filled type detector Gas molecules get ionized when an energetic charged particle passes through Electrons produced during ionization, if accelerated by high potential difference, can cause further ionization of the gas molecules, thereby enhancing the signal.

(12) GM Counter GM Tube The Geiger-Muller tube is a hollow cylinder typically of length 15 – 20 cm and made of copper The tube is filled with inert gas, generally with argon gas, at a pressure round 10 cmof Hg with 10% of ethyl alcohol vapour GM tube is enclosed in an evacuated glass tube.

(13) GM Counter GM Tube A tungsten wire is fixed along the axis of the GM tube (insulated from tube) The wire is connected to the +ve terminal and metallic body of the GM tube to the –ve terminal of a high-voltage source (few kV) A thin window (generally made of mica) at one end of the tube allows the radiation to enter the tube.

(14) GM Counter Principle of Operation When an energetic charged particle enters GM tube through the mica window, the gas molecules get ionized due to interaction with the incident charged particles The generated electrons get accelerated towards the central anode (the tungsten wire) & positive ions towards the cathode (the wall of GM tube) The accelerated electrons on their path can cause ionization of the gas molecules, generating large number of electrons within a very short time interval (avalanche).

(15) GM Counter Principle of Operation The avalanche gives rise to a detectable current pulse The pulse is amplified before it is converted into a TTL pulse The TTL pulse is fed into a counter For each particle entering the tube, successive current pulses are produced and counting is done.

(16) GM Counter. https://www.youtube.com/watch?v=PIsWy2q0hVc.

(17) GM Counter Plateau of GM counter Count rate (CR) of a GM counter is defined as the number of ion-counts per second The graph showing the variation of count rate with the anode voltage is called the characteristic of the GM counter For low voltage, the counter operates in the ionization chamber region where there is no amplification/avalanche. Detector signal is too small to process electronically A minimum voltage Vs, the threshold voltage is required for enabling the counter.

(18) GM Counter Plateau of GM counter As the voltage is increased beyond Vs, the gas amplification sets in and output pulse size increases gradually –. D. C. this is proportional counting region where more and more low energy particles are detected until the point C is reached. Vs.

(19) GM Counter Plateau of GM counter From the point C onwards, the counting rates become almost constant. The portion CD is called the plateau of. D. C. the counter where the counter records the incident particles irrespective of their energy. The GM counter is operated in this region. Vs.

(20) GM Counter Dead Time & Recovery Time During the working of a GM counter, the heavier +ve ions take larger time to reach the cathode. The next particle can not be detected until the +ve ions reaches the detector or they are neutralized The time interval between the production of the initial pulse and initiation of the second Geiger discharge is defined as the dead time of the GM. It is usually ~50 to 100 μs.

(21) GM Counter Dead Time & Recovery Time The actual resolving time of a counter is somewhat longer than the dead time, since a finite pulse must develop before it can be counted by the counter circuit (two terms, however, often interchangeable) The recovery time of the counter is defined as the time interval after which the counter returns to its original state to produce the full sized pulse again.

(22) GM Counter Dead Time & Recovery Time. The tube can produce no further pulses during the dead time, and only produces pulses of lesser height until the recovery time has elapsed.

(23) GM Counter True Count Rate The counter does not respond to all the ionizing events occurring in it for finite value of the resolving time τ N = No. of particles that enter the GM tube per second n = Count rate (the no. of particles actually counted per second). Obviously, n < N The interval per second during which the counter does not respond = nτ The number of particles incident within this interval = Nnτ = N – n. Nnτ = N − n n ⇒N= 1 − nτ. True Count Rate.

(24) GM Counter Quenching Just on the completion of dead time the slow moving +ve ions reach the surface of the cathode tube and get discharged there. As a result a current pulse is again generated, that gives an indication, as if another particle has entered the GM tube, which is not the case in reality A single particle, thus counted twice (one at the start & other at the end of the dead time interval) It is desirable that the +ve ions sheath around the central anode wire must be eliminated before they reach the cathode tube.

(25) GM Counter Quenching The process of eliminating undesired +ve ions sheath around the central anode wire in GM tube is called quenching Some halogen gas is introduced along with inert gas in GM tube for quenching.

(26) GM Counter Limitations Because the output pulse from a GM tube is always the same magnitude regardless of the energy of the incident radiation, the tube cannot differentiate between radiation types Inability to measure high radiation rates due to the "dead time" of the tube. This is an insensitive period after each ionization of the gas during which any further incident radiation will not result in a count, and the indicated rate is therefore lower than actual. Typically the dead time will reduce indicated count rates above about 104 to 105 counts per second depending on the characteristic of the tube being used.

(27) GM Counter Problem 1. A GM counter as a dead time of 400 μs. What is the true counting rate when the observed rate is 1000/minute? [CU – 2016]. Dead time τ = 400 μs, Observed count rate n = 1000 / minute n 1000 True count rate N = = = 1006.7 / minute −6 1 − nτ 1 − 1000 × 400 ×10 / 60.

(28) Problem. GM Counter. 2. An organic quenched GM tube operates at 1 kV and has a wire of diameter 0.2 mm. The radius of the cathode is 20 mm and the tube has warranted lifetime of 109 counts. What is the maximum radial field? How long will the counter last if it is used on the average for 30 hours per week at 3000 counts per minute? [CU – 2017] V The radial field Er = b r ln  a b = radius of the cathode = 20 mm, a = radius of the anode = 0.1 mm For maximum field r = a = 0.1 mm = 10−4 m V 103 Hence, the maximum radial field Er max = = = 1.89 × 106 V b  20  a ln  10−4 × ln  a 0 . 1     Total counts per week = 3000 × 30 × 60 = 5.4 × 106. Total lifetime count = 109 109 Hence, lifetime of the counter = = 185.19 weeks 6 5.4 × 10.

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