Applications of Accelerators: Industrial Applications of high

energy X-rays G. Burt, Lancaster University

X-ray production

• X-rays can be produced by bremstrahlung in a dense target from an electron beam.

• As the electrons are decelerated by Coloumb forces in the target nuclei they loose energy and radiate X-rays.

• The higher the nuclei charge the greater the force and the more efficient the interaction hence high Z targets are preferred.

• A lot of heat is also generated as not all interactions generate X-rays so a high melting point and thermal conductivity is preferred.

• A standard target material is tungsten, but gold, platinum and tantalum are also used.

Target

• Each electron is not necessarily decelerated in each interaction and multiple scattering occurs.

• This means a range of X-ray energies are produced from low energy up to the electron beam energy.

• Very low energies are often attenuated in thick targets.

Intensity

• The intensity for a given electron beam is normally proportional to the target Z.

Angular distribution • The electrons

undergo multiple scattering hence the X-ray direction is not entirely in the beam direction.

• High energy electrons are scattered less so the X-rays are directed in the electron beam axis.

• High Z targets have more efficient interactions so multiple scattering is less likely.

Applications of X-rays

• Radiotherapy (Hywel will cover this) • Sterilisation • Non-destructive testing • Cargo screening

X-ray attenuation

• X-rays will be attenuated as the pass through any medium dependant on the X-ray energy.

• The X-ray energy remains constant but some photons will be absorbed reducing the intensity.

• In the energy range used for scanning (1-9 MeV) absorption is dominated by compton scattering and pair production.

X-ray and EB sterilisation

• For sterilisation X-rays can also destroy bacteria and pathogens. • The X-rays penetrate further into the material than electrons. • Radioactive sources generating gamma’s can also be used but you

then need a source.

Penetration in water

Absorption • Attenuation is material dependant. • The mass attenuation coefficient

(attenuation per unit length, and density) is roughly constant with material for a few MeV.

• This means if we measure attenuation of a known signal through something of known length we get the material density.

• If not we get the density*length product which is still useful

Non-destructive testing (NDT) • One use of this is non-destructive

testing of large or dense structure.

• This can be used to look at defects/damage in bridges, power plants, reactors, pipelines, ships or engines.

• This used to be done with cobalt, cesium or iridium gamma sources but are now being replaced by linacs from 1-15 MeV due to higher energy, larger dose-rates and less security risks.

• You basically look for regions that are attenuated more or less than their surroundings.

Boeings aircraft battery that was catching fire

Cargo screening

• A major use is in cargo screening to ensure that what is in a shipping container is what is in the manifest and not cash, drugs, gold, uranium, cigarettes or cars.

Detector Array

In security screening we want a 2D image An array of scintillators and photodiodes are used to get the vertical resolution

Linac and Array set-up

Horizontal Resolution

The horizontal resolution is created by moving the object or the linac in time. • For trucks they go through the scanner at a few mph • For air cargo they go on a conveyor • For shipping containers the gantry moves

Different Solutions

Image courtesy of Rapiscan

Speed Limits

As the speed of the scintillator is limited, the maximum speed for sufficient resolution for a single device is 5kph For faster speeds more columns can be added.

M10 (and M45)

Why X-band?

• For a mobile linac mounted on a robotic arm the weight of the linac is critical.

• While the linac isn’t very big or heavy the shielding is.

• X-band means that the shielding diameter is much less.

• Area of shielding is given by • (2rcavtshield + tshield

2)π

• Availability of 9.3 GHz magnetrons

Energy requirements • As the X-rays are

attenuated we have to match energy with the cargo.

• We want the attenuation to be within 10%-90% to get a clear image.

• Depending on the cargo we need between 1-9 MeV.

• This is perfect for an RF linac operating at S-band or X-band.

Siemens SILAC 6-9 MeV scanner Unlike medical linacs most security linacs are side-coupled standing wave linacs. To improve reliability the gradient is only 10-20 MV/m

As well as the RF structure the unit also contains a magnetron (RF source), vacuum pumps, modulator (for the magnetron), HVDC power supply, an electron gun, a target and water cooling.

Linac Cavity • As this is a commercial

product it is simpler to have a single self contained electron gun, cavity, RF window, target and vacuum system.

• These are welded together to create a single sealed unit.

• The gun is typically 15-50 keV thermionic gun at 200 mA.

• Linacs normally are limited by the RF power source to 2-5 µs pulses at 100-500 Hz

π/2 mode structures • π/2 modes are more tolerant to frequency errors (due to

manufacturing tolerances) than π mode structures. • We can adjust the cell length to make the π/2 mode synchronous

using

• But the mode still has a low efficiency as every 2nd cell is unfilled

1a n cd ϕ βω

−=

π-mode

π/2-mode

high shunt impedance

high field stability

Side coupled or bi-periodic cavity

π-mode

π/2-mode

π/2-mode Side coupled

high shunt impedance

1. Beam sees a π-mode high acceleration 2. RF sees a π/2-mode field highly stable

high field stability

both !

We can improve the shunt impedance by making the coupling and accelerating cells have different geometries for π/2 mode structures.

L

π/2-mode bi-periodic

both !

π/2 mode stability • Why is a π/2 mode so stable • Each mode in the passband contributes a small error term to the

excitation of any one particular mode when there are frequency errors. Having a larger mode spacing, by increasing k, reduces this error term.

• For the π/2 mode the error terms for the higher frequency modes almost exactly cancels the contributions from the lower frequency modes.

• For this reason most long standing wave structures work in the π/2 mode.

0.94

0.96

0.98

1

1.02

1.04

1.06

0 2 4 6 8 10

Freq

uenc

y

Mode No.

Cavity geometry • The shunt impedance is

strongly dependant on aperture

Figures borrowed from Sami Tantawi

Similarly larger apertures lead to higher peak fields. Using thicker walls has a similar effect. Higher frequencies need smaller apertures as well

Nose-Cones

( ) ( )/ 2

/0

/ 2

, cosL

i z cz z

L

V E z t e dz E LT tω ω+

−

= ℜ =

∫

If we decrease the accelerating gap, while keeping the same voltage between the gap, the effective accelerating voltage increases due to the transit time factor.

However if we just use a small gap we get high magnetic fields on the walls which decreases R removing the benefit of increasing T.

Instead we can just decrease the gap near the beam by using nose-cones

As the gap is smaller the field strengths must increase for the same voltage, which is good where the beam is but bad elsewhere.

Applications where power loss is important use nose-cones but applications where gradient is important do not. Electric field

CI-SAC Nov 2010

RF bunching and focusing

Solenoids and quadrupoles are unacceptable for compact applications. Hence RF focusing from the linac structure is used

1. Initially the DC bunch see’s all phases, bunching phases are captured 2. The captured bunch then is accelerated moving towards the peak acceleration 3. The bunch is then moved to a radially focussing phase until the linac exit which

unfortunately starts to debunch the beam.

Radial focusing (long. Debunching)

-100

-80

-60

-40

-20

0

20

40

0 1 2 3 4 5 6 7 8

Cell

Phi, d

eg

1 MeV Buncher/Accelerator β = 0.28 0.48 0.68 0.88 1 1 1

17 keV

0

0.5

1

1.5

2

2.5

0 0.02 0.04 0.06 0.08 0.1 0.12

Xrm

s, m

m

Z, m

cavity ON

cavity OFF

0

0.2

0.4

0.6

0.8

1

1.2

0 0.02 0.04 0.06 0.08 0.1 0.12

Ekin

,MeV

Z, m

cavity ONcavity OFF

Z vs Radius Z vs Energy

Typical Beam distributions

The beam typically is very long, with a large spread in energy. Its not really an issue as its going to hit an X-ray target. As we have seen the X-rays have a large spread anyway.

Typical Beam distributions Radial divergence

Kinetic energy

The energy spread is quite large, but its not Gaussian it has a long tail. Typically energy spread is 5% FWHM and 20% standard deviation from the mean.

Material Separation • Different materials have slightly

different attenuation coefficients. • If we do a measurement at a single

energy we get the product of the attenuation coefficient and the path travelled, two unknowns.

• Hence we need two different measurements to resolve both, this is normally done by using two different X-ray energies.

• R is the ratio of mass attenuation coefficients, µ, given by

• Where I is the intesity with the cargo in place and I0 is the intensity with no cargo for energy 1 and 2.

( )( )

02 22

01 1 1

log /

log /

I IR

I Iµµ

= =

R for 5 and 9 MeV

R for 5 and 9 MeV filtered with lead

Broad spectrums

• Linac X-ray sources are not monochromatic so rather than using the attenuation at a single energy we have to use.

( ) ( ) ( )

( ) ( )

max

max

0 0

0

expE

E

E E X r EI I

E r E dE

φ µ

φ

=

∫

∫

Where φ is the energy spectrum, r is the relative detector response

Dual Energy •Typically the linac will produce two interleaved energies. •The energy is varied by varying the power supplied from the magnetron. •This means the current from the modulator needs to vary, this takes time to switch so the pulses have to be at least 50 µs apart.

Dual Energy Images The R value is dependant on Z so we can designate colour with Z on the images. High Z materials are red low Z is blue.

G60 Materials Separation

Steel

Plastic Aluminium

G60 Materials Separation

Steel

Plastic Aluminium

Images

Neutrons • Neutrons can also be used for cargo

scanning. • A pulse of neutron particles, for

example, would illuminate the items inside the cargo and react with the nuclei, which then emit gamma rays. A particle detector would identify the type of gamma ray and reveal what is inside the box.

• As such neutrons can also be used for material discrimination but the variation in neutron energy from most sources is a real challenge.

• Better discrimination is obtained by combining a neutron source with an X-ray or gamma source.

• The neutrons can come from sealed sources, reactors or in our case an accelerator (typically an RFQ)

Comparison • As can be seen the R values of

combined neutron/gamma systems is far better than for X-ray systems alone.

• The main application of this is the detection of organic-based explosives and drugs.

• Neutrons are far better at this.