LECTURE 15 EXPERIMENTAL METHODS: ACCELERATORS PHY492 Nuclear and Elementary Particle Physics

LECTURE 15
EXPERIMENTAL METHODS:
ACCELERATORS
PHY492 Nuclear and Elementary Particle Physics
Term Paper Milestones
Select Topic
: Feb 14 (Friday)
Outline Due
: March 10 (Monday)
First draft due : March 26 (Wednesday)
Final draft due : April 21 (Monday)
Missing a paper milestone will result in a penalty of 2% for the
total paper grade (out of 25% total possible). Eg, missing the
outline milestone means you can get a maximum of 23% instead
of 25%.
If you do not want to select your own topic, let me know before the
deadline and I will assign one to you.
February 12, 2014 PHY492, Lecture 15 2 Experimental Methods
What is the best experimental method ? A For example, the measurement of nuclear lifetimes
one can observe the activity A as a function of time
A (t) = λN0 exp(-λt)
A(0) A(0)/2 T1/2 But ….
t decay of proton : lifetime > 1032 years 1 proton February 12, 2014 To observe the decay,
one needs to wait
for about 1032 years,
… or One needs to
collect 1032 protons
(water 107 liters!)
and observe for 1 year
1032 protons …
PHY492, Lecture 15 3 Center-of-momentum Energy
To explore the structure of nuclei (nuclear physics) or
hadrons (particle physics) requires projectiles whose wave length
at least as small as the size of nuclei or hadrons.
accelerators
To produce new and (more) unstable particles, large center-of-momentum energies
are required.
Ecm2 = (p
Fixed target beam
+p
target)2
c2 Ecm2 = ( p beam2 + p target2 + 2 p beam p target ) c2
= ( mb2c4 + mt2c4 + 2mt c2 EL )
p beam
p target
= (EL/c,pb) = (mt c,0) Colliding beam Ecm increases like EL1/2 Ecm2 = ( p beam12 + p beam22 + 2 p beam1 p beam2 ) c2
= ( EL2 – pb2c2 + EL2 – pb2c2 + 2EL2 + 2pb2c2 )
p beam1
p beam2
= ( 2EL ) 2
disadvantage:
= (EL/c,pb) = (EL/c, -pb) Ecm scaled by 2EL low luminosity February 12, 2014 PHY492, Lecture 15 4 DC accelerators
The earliest type of DC accelerator was the Cockroft-Walton machine,
in which ions are accelerated though sets of aligned electrodes operated
at higher potentials.
February 12, 2014 PHY492, Lecture 15 5 DC accelerators
The earliest type of DC accelerator was the Cockroft-Walton machine,
in which ions are accelerated though sets of aligned electrodes operated
at higher potentials.
Can achieve up to E = ~1 MeV February 12, 2014 PHY492, Lecture 15 6 DC accelerators
The earliest type of DC accelerator was the Cockroft-Walton machine,
in which ions are accelerated though sets of aligned electrodes operated
at higher potentials.
The “tandem” Van de Graaff accelerator
can double the energy of the simple machine
Up to E = ~40 MeV 1. a high voltage source at I passes positive ions to a belt
via a comb arrangement at C
2. ions are carried on the belt from the first pulley (P) to the second,
and sent to another comb in a metal vessel T
3. The charges are transferred to the outer surface which acts as an
external terminal
4. Singly-charged negative ions are injected from a source and accelerated
along a tube toward T, and two or more electrons are removed in a stripper S.
The positive ions can be further accelerated (tandem).
I February 12, 2014 PHY492, Lecture 15 7 AC accelerators: Linac
In a linear accelerator (Linac), particles pass through a series of metal
pipes (drift tubes) which are connected successively to alternate terminals
of an rf oscillator.
CEBAF a variety of linacs in USA
-  ATLAS ( Argonne National Lab.)
first superconducting linear accelerator
7-17 MeV/nucleon stable, unstable ions
-  SLC ( SLAC lab. In Stanford)
cylindrical metal cavity + magnetic focus lens
50GeV electron　- 3km
-  CEBAF ( Jefferson Lab.)
two short linacs with bending lines, 0.5-6.0GeV intense electron
February 12, 2014 PHY492, Lecture 15 8 AC accelerators: Linac
February 12, 2014 PHY492, Lecture 15 9 AC accelerators: Cyclotron
For a low-energy nuclear physics experiments,
cyclic accelerators, called cyclotrons, are used.
They are also used to produce beams for
Medical application, including proton beam
radiation therapy.
1.  Charged particles are injected into the machine
near its center.
2.  Magnetic field bends path of charged particles.
3.  The ions are accelerated each time they pass
across the gap between the dees with an rf field.
Cyclotron frequency ( ωc ) mv2
= qvB
B r
qBr
v = v m
q 2πm
t = 2πr/v = qB
February 12, 2014 PHY492, Lecture 15 ωc
= 2πf
= 2π/T
qB
= m
10 Neutral and Unstable Particle Beams
The particles used in accelerators must be stable and charged,
but ,there are a number of ways to produce beams of
neutral (neutron, photons,…) or unstable (exotic nuclei, π±,…) particles. neutrons nuclear reactor : low energies of 1-2MeV
ILL reactor, Institute Laue-Langevin, in France
spallation : ISIS, Rutherford Appleton Laboratory, in UK
Linac
Synclo.
70MeV 800MeV proton Ta target neutron unstable particles proton
heavy ions February 12, 2014 Heavy target PHY492, Lecture 15 secondary beams
( π±, µ±, exotic nuclei, …) 11 AC accelerators: Synchrotron
Cyclic accelerators used in particle physics are called synchrotrons.
The operation is similar to that of a linear accelerator, but synchrotrons
have a near circular orbit. Acceleration is achieved as the beam repeatedly traverses one or more
cavities placed in the ring where energy is given to the particles.
Synchrotrons are often used as colliders.
For a relativistic particle of mass m, there is a significant energy loss due to
synchrotron radiation, which is scaled by 1/m4. useful relation Power lost “per turn” mv2
= qvB
r
𝑃= ​2𝑞2𝑐𝛾4/2(4𝜋𝜀)𝑟2 = qBr p = mv
transformation
from SI units , 𝛾= ​𝐸/𝑚𝑐2 p [GeV/c] = 0.3 q B [T] r [m] February 12, 2014 PHY492, Lecture 15 12 AC accelerators: Synchrotron
Cyclic accelerators used in particle physics are called synchrotrons.
The operation is similar to that of a linear accelerator, but synchrotrons
have a near circular orbit. February 12, 2014 PHY492, Lecture 15 13 High Energy Accelerator Chains
LHC @ CERN February 12, 2014 PHY492, Lecture 15 14 High Energy Accelerator Chains
LHC @ CERN ➡ Circumference: 27 km ➡ Current proton kineDc energy: 4 TeV ‣99.999997% of speed of light ‣8 m/s slower than light ➡ Current total energy in beam: 135 MJ ‣Equivalent to an aircraU carrier moving at 3.8 MPH ‣Or calories in seven 8” Cold Stone Creamery Cheesecakes Named Desire ➡ SuperconducDng magnet temperature is 2 K (colder than space) ➡ Colliding protons like shooDng two needles at each other from a distance of 6 miles and having them hit in the middle February 12, 2014 PHY492, Lecture 15 15 High Energy Accelerator Chains
LHC @ CERN February 12, 2014 PHY492, Lecture 15 16 High Energy Accelerator Chains
LHC @ CERN February 12, 2014 PHY492, Lecture 15 17 High Energy Accelerator Chains
LHC @ CERN February 12, 2014 PHY492, Lecture 15 18 Rare Isotope Beam
In-­‐Flight Method Heavy ion beams ProducDon target @NSCL/MSU Fragment separator Fast RI beam Heavy ion accelerator RadioacDve ion beam E>Fermi energy ~ 30A MeV projecDle fragmentaDon fission Experiment ISOL Method Transfer tube Ion source Isotope/isobar Slow RI beam separator RadioacDve ion beam very low energy ProducDon accelerator ProducDon target target fragmentaDon or spallaDon fission Experiment Postaccelerator RadioacDve ion beam E < Coulomb barrier ~ 10A MeV February 12, 2014 PHY492, Lecture 15 Experiment 19