Dr. Abdul Ahad

An Overview of 5MV
Pelletron Accelerator at
NCP
Dr. Abdul Ahad

Contents
• Electrostatic Accelerators
• 5UDH-2 Pelletron at NCP
– introduction
– Control System
– Ion Sources
– RC43 Setup
– RS61 Setup
• Applications (Ion Beam Analysis)

Electrostatic Accelerators
• True DC
• Low Energy Spread
• Current
• All Masses
100% duty cycle
100e V to few KeV
(1 MeV to 100’s s MeV)
nano to micro amp
(Mass independent)
3

Electrostatic Accelerators
Single Ended
• Higher Currents
• Ne, Ar, Xe
• Less energy spread (microprobes)

Electrostatic Accelerators
Tandem
• Source, etc. outside accelerator tank
• No molecular interferences - He++ vs. H2+
• Flexibility – Multiple sources
• Easy to upgrade
• Higher E for Vt, especially for heavy ions
THE NCP 5UDH-2 IS A TANDEM

Electrostatic Accelerators
Belt Charging system
Current transported by belt
Characteristics of Ideal Belt
- high resistivity
- little stretch
- moisture resistant
- smooth surface
- sufficient mechanical strength
• Dust Generation

Electrostatic Accelerators

Electrostatic Accelerators
• Pelletron charging System By NEC (1960)
• Metal cylinders with rounded ends joined by nylon
• Move at at bout 15 m/s
• Delivers charging currents of 100 - 200 µA
• motors are supported on movable platforms which
are counterweighted, automatically providing proper
chain tension

Electrostatic Accelerators
Pelletron Chain
• Spark damage well
protected.
• Better voltage stability.
- High efficiency.
- Insensitive to moisture.
- Long life (50,000 hours).
- Simple and reliable.
- Go to over 30 MV

Pelletron Charging
11

The NCP 5UDH-2 2 PELLETRON
• 5 MV Terminal Voltage
• Dual Ion Sources
– SNICS II for wide range of light and heavy ions
– RF-Charge Exchange for He
• Beam currents
– 5 – 10 microamps for most probable charge states,
– 100's of pA for high charge state heavy ions
• Energies
– 10 MeV for Protons, 15 MeV for He++
– Useful currents for heavy ions to over 50 MeV
• 15 degree beamline for Materials Analysis
• 30 degree beamline for Ion Scattering/Nuclear Physics
13

The NCP 5 MV Pelletron

The NCP 5 MV Pelletron
15

The NCP 5 MV Pelletron
16

The NCP 5 MV Column
17

Control system
Control & Data
Acquisition
• LINUX based
computer user
interface
• MCA CARDS for data
acquisition
18

Control system

Beam Energy
• Tandem accelerates the negative ion to the
positive terminal
• Then strips to many charge states, Q
• Each state picks up Q x MV
• Total Energy = (Q+1)MV
• Charge state probability depends on terminal
voltage, ion mass, and stripper
20

Beam Currents
• The next slide shows the wide variety of
beam energies and currents from a
tandem at 3MV
• Higher energies are possible in the 5UDH
21

ION SOURCES
• The purpose of the ions sources are to
produce either positive or negative ions
from neutral atoms of elements of interest.
• the ions beams are injected to the
accelerator tube for gaining high energy.
23

SNICS II Ion Source
• “Source of Negative Ions by Cesium
Sputtering” from solid materials.
• Cs ions from hot “ionizer” surface are
accelerated to the cathode.
• Cathode contains element (s) for beams.
• Simple operation; rapid cathode change.
• Currents up to 100's of micoramps
depending on material.
24

SNICS II Ion Sources

Principle Of Operation
• Cs vapor comes from the cesium oven into
an enclosed area between the cooled
cathode and the heated ionizing surface.
• Some of the cesium condenses on the
front of the cathode and some of the
cesium is ionized by the hot surface.
26

Cont..
• The ionized cesium accelerates towards the
cathode, sputtering particles from the cathode
through the condensed cesium layer.
• Some materials will preferentially sputter
negative ions.
• Other materials will preferentially sputter neutral
or positive particles which pick up electrons as
they pass through the condensed cesium layer,
producing negative ions.
27

SNICS II Ion Sources
28

Alphatross Ion Sources

RF Charge Exchange Source
30

Operation Principle
• There are two main components in this device.
The first one is the radio frequency positive ion
source.
• Helium gas is bled into a quartz discharge bottle
through a metering valve to maintain a pressure
of 10-50 millitorr.
• An RF oscillator induces a plasma in the bottle
which is intensified by the solenoid magnet.
31

Cont..
• A DC potential is applied across the
plasma by probe power supply.
• This potential extracts ions from the
plasma and accelerates them through the
Ta exit canal.
• Then they enter the charge exchange
chamber.
32

Cont..
• The charge exchange is the second main
component of the Alphatross.
• In it He + ions are neutralized by Rb vapor.
• A few He atoms then undergo a 2 nd charge
exchange reaction and become negative
He ions.
33

RF Charge Exchange Source
Schematic
34

15 Degree NEC RC43 Setup
• RBS with channeling
• Elastic Recoil Detection (ERD)
– Surface hydrogen analysis
• Nuclear Reaction Analysis
(NRA)
– Element specific analysis
– E.g.:
• 15N + H (Hydrogen depth
profiling)
• 4He + 16O (enhanced
oxygen detection)
• Particle Induced X-Ray
Emission (PIXE)
• Particle Induced Gamma
Emission (PIGE)
– Trace element analysis
35

RC43 RBS Data Collection
37

NEC RS61 Scattering System
• Ion Scattering Studies
• Nuclear Physics
• Atomic Physics
• Ion Implantation
• Radiation Damage
Studies
38

5UDH-2 2 APPLICATIONS
40

Ion Beam Analysis

Ion Beam Analysis
• The 5UDH-2 accelerator systems is equipped for materials analysis with ion
beams at MeV energies, using one or more of the following analytic
techniques:
• RBS (Rutherford Backscattering): for elemental depth profiling
• Channeling: for measuring crystalline perfection and impurities in crystals
• ERD (Elastic Recoil Detection): primarily for Hydrogen depth profiling
• NRA (Nuclear Reaction Analysis): for measuring selected isotopes with
enhanced sensitivity
• PIXE (particle Induced X-Ray Emission): for trace element analysis

Rutherford Back Scattering (RBS)
• Rutherford scattering is simple, central force,
elastic scattering.
• The effects of nuclear reactions are avoided by
operating at beam energies below the Coulomb
barrier.
• This is basically billiard ball scattering.
Kinematices and absolute cross sections can be
calculated directly from first principles.

Rutherford Back Scattering (RBS)
• The thickness of the surface layer is determined
from the energy width of the surface peak, using
standard dE/dx energy loss calculations.
• The elemental concentration as a function of
depth can also be determined from the number
of counts as a function of energy for both the
surface layer and the substrate.
• Channeling Measures crystal perfection, which
is especially valuable in silicon technologies.

PIXE
• Non-destructive, quantitative analysis of heavier
elements.
• Generally used for trace element analysis 1-100ppm.
• Identify high-Z elements (11

PIXE
• Protons at an energy of a
few MeV are used to
excite the atoms in a
sample.
• The excited atoms emit
characteristic X-rays.
• The energy of the X-ray
identifies the element
from which it came, and
the number of X-rays of a
given energy is a
measure of the element’s
concentration.

PIXE
• Probed depth tens of µm.
• Sensitivity up to 1ppm.
• Accuracy 10%.
• Higher signal to background ratio as
compared to electron beam induced x-ray
spectroscopy.
• Low velocities changes as compared to
electron, hence no bremsstrahlung.
• PIXE is 100 times more sensitive than EDX.

Thanks
48

Control & Data Acquisition
• Control & Data
Acquisition
• LINUX BASED COMPUTER
CONTROLLED SYSTEM
• MCA CARDS
• Data Analysis Software
• RUMP FOR RBS/ERDA
SPECTRUM ANALYSIS
• GUPIX FOR X-RAY
SPECTRUM ANALYSIS
• SIMNRA FOR NUCLEAR
REACTION ANALYSIS
49

Ion Beam Analysis

RBS
E o
E
E=KEo