Download The Synchrotron

The Synchrotron
•
•
•
•
•
•
•
•
•
•
Chapter
8
describe that an oscillating electron produces electromagnetic
radiation, and similarly, that electromagnetic radiation is produced
when the direction of motion of an electron changes since the changing electric field
produces a changing magnetic field;
analyse acceleration of electrons in a synchrotron in terms of
– applying a force in the direction of motion of the electron (due to an electric
field, F = qE)
– applying a force perpendicular to the direction of motion of the electron (due to a
magnetic field, F = qvB);
describe the production of synchrotron radiation by an electron radiating energy at
a tangent to its circular path;
analyse the acceleration of electrons in an electron gun, F = qV/d and ΔEk= qV;
analyse the magnitude and direction of the force applied to an electron beam by a
magnetic field, F = qvB in cases where the directions of v and B are perpendicular or
parallel;
calculate the radius of the path followed by a low-velocity electron in a magnetic
field, qvB = mv2/r;
describe the basic design of The Australian Synchrotron including the general
purpose of the electron linac (details about drift tubes and RF cavities are not
required), circular booster, storage ring and beamlines;
compare the characteristics of synchrotron radiation, including brightness,
spectrum and divergence with the characteristics of electromagnetic radiation from
other sources including lasers and X-ray tubes;
explain, using the characteristics of brightness, spectrum and divergence, why for
some experiments synchrotron radiation is preferable to laser-light and radiation
from X-ray tubes;
describe the production of light by insertion devices (dipoles, wigglers and
undulators) and the properties of the light produced in terms of relative brightness
and energy;
The charge on the electron is (–)1.6 × 10-19 C.
The mass of the electron is 9.1 × 10-31 kg.
1.
[3 marks]
(2004 Sample Q1)
X-rays are an important non-invasive diagnostic tool. Since their discovery, x-rays have had
wide application in medicine and science. For most of this time, x-rays have been produced
with small electron accelerators. In recent times, synchrotron radiation has emerged as a viable
and superior alternative for the production of x-rays.
Describe the characteristics of synchrotron radiation which make it superior to conventional xrays.
2.
[3 marks]
(2004 Sample Q3)
A schematic diagram of a cathode ray oscilloscope (CRO) is shown below. It consists of an
electron gun (filament) that boils off electrons into an evacuated space. The electrons are then
accelerated to a positively charged anode. The anode contains a hole for the electrons to pass
through.
The electron has a charge of 1.6 × 10-19 C and a mass of 9.1 × 10-31 kg.
Calculate the anode voltage (V) required to accelerate the electrons to a velocity of 5 × 107 m s-1.
3.
[3 marks]
(2004 Sample Q4)
The figure below is a section of a typical linear accelerator (linac). The linac consists of an
electron injector (gun) attached to a series of accelerating chambers. Electrons which emerge
from the injector with kinetic energy of 120 keV are accelerated to very high energies, 300 MeV
in this particular example.
Briefly explain how the electrons in a linac are accelerated to these high energies. Make sure
that you highlight in your answer what provides the high-potentials required for acceleration.
4.
[2 marks]
(2004 Sample Q5)
The diagram below, is a prototype design for a high-energy electron synchrotron ring. A linac is
used to accelerate the electrons which are then stored in the large ring. The storage ring is
constructed from eight dipole bending magnets, each of magnetic field strength 1.5 T. Between
each dipole magnet there is a straight section as indicated on the diagram. One of these sections
contains a small RF power source. Pumps are used to maintain the ring under vacuum.
Which one of the following statements (A – D) best describes why there is additional RF
acceleration in the straight sections of a synchrotron ring?
A.
The vacuum is not completely zero and so electrons lose energy as they travel through the
ring.
B.
The bending and focusing magnets consume energy.
C.
Electrons collide with each other and lose energy.
D.
Electrons lose energy because of synchrotron radiation.
5.
[3 marks]
(2004 Sample Q6)
At each of the bending magnets, the electrons are bent through an arc of radius 10 m. Use this
information, together with the characteristics of the magnet, to calculate the momentum of the
electrons in the ring.
6.
[4 marks]
(2004 Pilot Q3)
Identify two beneficial characteristics of the radiation produced by a synchrotron and explain
why these characteristics are beneficial.
7.
[2 marks]
(2004 Pilot Q4)
Each stage of a LINAC boosts the energy of an electron beam by the same amount.
However, the physical lengths of the accelerating stages are different. Why is this the case?
8.
[2 marks]
(2004 Pilot Q5)
A beam of 10 keV electrons enters a uniform magnetic field of strength 2.0T.
Which one of the following best describes the path of an electron as it passes through the
magnetic field above?
A.
the electron is deflected to the left
B.
the electron is deflected to the right
C.
the electron is deflected up
D.
the electron is deflected down.
9.
[3 marks]
(2004 Pilot Q6)
Show that the velocity for 10 keV electrons is approximately 6 × 107 m s-1.
(me = 9.1 × 10-31 kg)
10.
[3 marks]
(2004 Pilot Q7)
Determine the radius of curvature of 10 keV electrons as they travel through a magnetic field of
2.0T.
11.
[3 marks]
(2005 Q1)
In the sentences below, options are given within the brackets. Only one of the options will be
correct. Circle the best option.
In a synchrotron, the circulating electrons are produced in
[an electron gun / a linac / a storage ring]. Ultimately the electrons are
accelerated to a speed of approximately [50% / 90% / 99.99%] of the speed of
light by [a linac / a storage ring / an undulator].
12.
[2 marks]
(2005 Q2)
One section of a storage ring is shown below. Electrons travelling through this section of the
storage ring have a momentum of approximately 1.2 × 10-18 kg m s-1 and are bent through an arc
of radius 7.7 m as shown. The charge on the electron is .1.6 × 10-19 C.
Calculate the strength of the magnetic field required to keep the electrons on this arc.
13.
[2 marks]
(2005 Q6)
Choose one of the following options to complete the sentence.
Radiation generated by a synchrotron occurs because electrons
A.
have high energies.
B.
accelerate when they change direction.
C.
collide with other electrons.
D.
collide with residual air particles.
14.
[2 marks]
(2005 Q7)
A typical cathode ray tube is shown below. It consists of an electron gun, a deflecting system
and a fluorescent screen that emits light when struck by electrons.
The electron gun consists of a filament that emits electrons with very small kinetic energy into
an electric field created by a high voltage, V, applied between a pair of parallel plates. In a
particular case the electrons emerge from the gun with an energy of 8.0 × 10-16 J.
Charge on the electron: e = .1.6 × 10-19 C
Calculate the voltage, V, between the plates, used to accelerate the electrons.
15.
[2 marks]
(2005 Q8)
After acceleration, the electrons enter the magnetic-deflecting system which consists of two
pairs of mutually perpendicular magnetic-field coils (W and X), (Y and Z) aligned as shown
above. The electrons are deflected downwards, as shown.
Choose one of the following options to complete the sentence.
The downward deflection can be achieved by the coils
A.
WX producing a magnetic field in direction W to X.
B.
WX producing a magnetic field in direction X to W.
C.
YZ producing a magnetic field in direction Y to Z.
D.
YZ producing a magnetic field in direction Z to Y.
16.
[3 marks]
(2006 Q1)
A student is preparing a brief talk on the operation of the Australian Synchrotron Facility. In the
speech below, options to complete each sentence are given within the brackets. Circle the
correct option in each case.
In the Australian synchrotron, a beam of electrons is accelerated to almost the speed
of light by the [storage ring / linear accelerator / beam line]. The beam is then
confined to a circular orbit by a series of [RF cavities / bending magnets / beam
lines] separated by straight sections. As the beam is deflected, [monochromatic /
longitudinal / electromagnetic] radiation, which is commonly called synchrotron
light, is produced.
17.
[4 marks]
(2006 Q2)
It is claimed that synchrotrons provide a superior source of X-rays compared to those produced
by an X-ray tube.
Explain two advantages of using a synchrotron as a source of X-rays rather than an X-ray tube.
18.
[2 marks]
(2006 Q3)
In the Synchrotron Facility electrons are accelerated by several devices.
Which one of the following statements (A - D below) best describes the order in which this
acceleration occurs?
A.
linac  electron gun  booster ring
B.
electron gun  linac  booster ring
C.
linac  booster ring  electron gun
D.
electron gun  booster ring  linac
19.
[2 marks]
(2006 Q4)
Electrons are circulated in the synchrotron ring when they have reached their maximum
energy. To overcome energy loss in the circulating beam, additional energy is provided by RF
acceleration in one of the straight sections of the ring.
Which one of the following statements (A - D below) best describes the largest contribution to
this energy loss?
A.
The vacuum is not completely zero and so electrons lose energy as they circulate through
the ring.
B.
The bending and focusing magnets consume energy.
C.
Electrons collide with each other and lose energy.
D.
Electrons lose energy because of synchrotron radiation.
20.
[3 marks]
(2006 Q7)
A simplified set-up for an electron gun is shown below. It contains a high-voltage source (HV).
In normal operation, electrons emerge from the gun with energies of 5 keV.
Calculate the velocity for electrons of energy 5 keV.
(me = 9.11 × 10-31 kg)
21.
[3 marks]
(2006 Q8)
As high-energy electrons pass through one of the bending magnets in the Australian
Synchrotron Facility, they are bent through an arc of radius 7 m. The magnetic field strength is
1.7 T (Wb m-2).
Use the information above to determine the momentum of the electrons. Include a unit in your
answer. (Ignore any relativistic effects in your calculation.)
22.
[3 marks]
(2007 Q1)
In the paragraph below, options to complete each sentence are given within the brackets. Circle
the correct option in each case.
The booster ring of a synchrotron [ gives the initial boost to stationary
electrons / accelerates electrons from the linac / stores electrons until needed
in a beam line ].
Synchrotron radiation emerges as the electrons in the storage ring [ pass from
the storage ring into a beam line / travel around curved segments / travel at
constant speed along straight segments ]. Energy lost by the electrons to
synchrotron radiation is replenished by [ diverting the electrons through the
booster ring / electric fields in the straight segments / the bending magnets
in circular sections ].
23.
[2 marks]
(2007 Q2)
The figure below shows a simple electron gun used to inject electrons into the linac of a
synchrotron. The accelerating voltage is 2000 V.
Mass of electron = 9.11 × 10-31 kg
Charge on electron = -1.60 × 10-19 C
At what speed do the electrons emerge from the electron gun? (Ignore relativistic effects.)
Use the following information to answer Questions 25 and 26.
24.
[2 marks]
(2007 Q3)
A magnet called an injection magnet of magnetic field strength 1.2 × 10-4 T bends a pulse of
electrons emitted from the linac so that they enter the circular booster ring as shown below.
The electrons emerge from the linac with a speed of 5.0 × 106 m s-1.
What is the magnitude of the radius of the trajectory of the electrons from the linac as they pass
through the injection magnet?
25.
[2 marks]
(2007 Q4)
In normal operation, the injection magnet switches on only for the brief instant necessary to
direct the pulse of electrons into the circular booster ring.
However, due to a malfunction, the magnet was still on after the pulse of electrons had
completed one revolution of the circular booster ring. The electrons now pass through the
magnet a second time.
Which one of the following (A - D) would occur?
A. The electrons would continue around the ring.
B. The electrons would be deflected outwards from the ring.
C. The electrons would be deflected inwards from the ring.
D. The electrons would lose energy and continue around the circular booster ring at a lower
speed.
26.
[3 marks]
(2007 Q5)
In a synchrotron, an electron which is travelling at 2500 m s-1 moves from a linear region into a
magnetic field of 0.50 T as shown below.
What will be the magnitude and direction (into page, out of page, up page, down page) of the
force on the electron?
27.
[2 marks]
(2007 Q6)
In a typical synchrotron experiment to study a sample of material, which one of the following
(A–D) passes along a beam line?
A. a beam of energetic electrons
B. a strong magnetic field
C. a beam of electromagnetic radiation
D. a beam of laser light
28.
[2 marks]
(VCE 2008 Q1)
In the electron gun of a synchrotron, electrons are accelerated by a voltage of 5000 V.
Which one of the following best gives the speed of the electrons?
A.
1.8 × 1015 m s-1
B.
4.2 × 107 m s-1
C.
3.0 × 108 m s-1
D.
5.0 × 103 m s-1
Use the following information to answer the next two questions.
29.
[2 marks]
(VCE 2008 Q2)
In a particular part of the synchrotron, electrons are maintained in a circular path by a magnetic
field. The electrons are moving at 4.00 × 107 m s-1, perpendicular to a magnetic field of strength
7.60 × 10-4 T.
Which one of the following best gives the force on each electron?
A.
3.64 × 10-23 N
B.
4.86 × 10-17 N
C.
4.86 × 10-15 N
D.
1.26 × 10-12 N
30.
[2 marks]
(VCE 2008 Q3)
Which one of the following best gives the radius of the electron’s path?
A.
0.30 m
B.
3.0 m
C.
4.6 m
D.
34 m
31.
[2 marks]
(VCE 2008 Q4)
Which one of the following sequences best gives the order in which electrons pass through the
synchrotron?
A.
electron gun, booster ring, storage ring, linac
B.
storage ring, linac, booster ring, beam line
C.
electron gun, linac, booster ring, storage ring
D.
linac, storage ring, booster ring, beam line
32.
[2 marks]
(VCE 2008 Q5)
Diffuse scattering of X-rays involves
A.
the broadening of the pattern due to thermal motion of atoms and electrons.
B.
diffraction as X-rays pass through the interatomic spacing.
C.
loss of energy by the X-rays to thermal motion of the electrons.
D.
gain of energy by X-rays from Compton collisions.
33.
[2 marks]
Thomson scattering involves X-rays
A.
interacting with electrons with an increase in wavelength.
B.
interacting with electrons with a decrease in wavelength.
C.
interacting with electrons with no change in wavelength.
D.
absorbing energy from thermal motion and electrons.
(VCE 2008 Q6)
34.
[2 marks]
(VCE 2008 Q7)
Compton scattering involves X-rays
A.
interacting with electrons with an increase in wavelength.
B.
interacting with electrons with a decrease in wavelength.
C.
interacting with electrons with no change in wavelength.
D.
absorbing energy from thermal motion of electrons.
35.
[2 marks]
(VCE 2008 Q11)
Electrons enter the booster ring with a speed of approximately 0.99 c.
In the straight sections of the booster ring, the electrons are acted on by high voltage RF electric
fields.
Which one of the following best describes the effect of these fields on the electrons?
A.
The kinetic energy and speed both increase substantially.
B.
The kinetic energy increases substantially but the speed increases only slightly.
C.
The kinetic energy increases slightly but the speed increases substantially.
D.
The kinetic energy and speed increase only slightly.