3 MV Tandetron

Description

The 3 MV Tandetron Accelerator, manufactured by High Voltage Engineering Europe B.V., was installed and commissioned in 2012 and is mainly dedicated to applied research [1]. This installation is fully equipped to perform studies of materials with accelerated ion beams (IBA - Ion Beam Analysis), ion implantations (IIB - Ion Implantation Beam-line) and radiation resistance tests for various types of advanced materials, as well as cross-section measurements (CSM-Cross-Section Measurements), of nuclear astrophysics interest. The 3 MV Tandetron has three experimental beam-lines, each with a reaction chamber containing all the necessary equipment:
- IBA studies: charged particle detectors for Rutherford backscattering spectrometry (RBS) and elastic recoil analysis (ERDA), SDD detectors for X-rays used in particle-induced X-ray spectroscopy (PIXE), semiconductor detectors (HPGe - High purity germanium) for γ radiation (PIGE) and Markus ionisation chamber used in the dosimetry required for radiobiology experiments. The IBA line also contains a set of four electrostatic quadrupoles, with the help of which the ion beam can be focused up to a diameter of the order of microns.
- implantation/irradiation experiments with ion beams: electrostatic ion beam sweep system in the XY plane (17x17 cm²), with four Faraday cups that can determine the fluency with an accuracy better than 98%.
- Cross-Section measurements beam-line (CSM) that can be used for various experiments. The line can be equipped with: HPGe detectors, Si detectors for charged particles, collimation system and Faraday Cups for optimizing ionic optics and accurate determination of beam electric current.

Lentilă electrostatică pentru microfascicul

3D CAD representation of the electrostatic lens used to obtain the microfeam: 1. support plate; 2. ceramic insulators; 3. quadrupole electrodes.[2]

Main subsystems of the 3 MV Tandetron accelerator

To make it possible to apply an acceleration voltage of up to 3 MV, the inside of the tank in which the acceleration column is immersed is filled with sulfur hexafluoride (SF₆) at a pressure of 6 bar. (SF₆) is strongly electronegative, absorbing free electrons produced by micro-discharges, the dielectric properties of this gas are thus 2.5 times better than those of air, having a dielectric constant at 25^oC and 1 bar of 1.00204. Being a tandem electrostatic accelerator, the high voltage is applied to the middle of the acceleration column, and its ends are connected to earth. The two sections of the acceleration column are made up of a succession of conductive cylinders (metal), separated by insulating cylinders (glass), glued tightly so that the pressure inside the column can be maintained in the range 10^-7 - 10^-5 mbar. The metal cylinders are connected to each other by means of resistors of 300 MΩ, thus creating a voltage divider with the role of ensuring a uniform electric field.

Schematic representation of the 3 MV Tandetron accelerator with indication of the main subsystems

Na charge exchange channel for α particle beam

1. Dual ion injector

Negative sputter ion source with Cs (model 860A):

- Electrode extraction
- Manually operated vacuum valve
- Einzel type electrostatic lenses
- Y-axis electrostatic deflector

Duoplasmatron ion source (model 358):

- Electrode extraction
- Sodium charge exchange canal
- Magnet analyzer at 90^o

Sodium furnace

2. T-shaped Tandetron accelerator for medium beam currents:

Q-snout electrostatic lens
Pressurized vessel
Acceleration column
Voltage divider
Gas stripping channel (Ar) and recirculation for medium beams
Electrode extraction Carousel for carbon stripping foils

Generator de înaltă tensiune (3 MV) de tip Tandetron

Tandetron type high voltage generator for medium current beam

3. Tandetron type high voltage generator for medium current beam

HV multiplier and rectifier
HV stabilization and control system with GVM (Generating Voltmeter)
Capacitive load handling system

4. Radiation protection screen for bremsstrahlung radiation

5. Set of three electrostatic quadrupoles

6. Deflector magnet (ports at: 10^o, 20^o, 30^o and 45^o)

Power supply with the possibility of changing the polarity
Faraday cup on the port of 0^o

7. Beam line and reaction chamber equipped for IBA studies

• Micrometre adjustable image slots along (X&Y) axis
• X-axis electrostatic deflector
• Y axis electrostatic deflector
• (X&Y)-axis micrometrically adjustable object slots
• Electrostatic quadrupole for micro-beam applications
• Electro-pneumatically operated Faraday cup
• Beam Profile Monitor (BPM)
• Cylindrical vacuum chamber (h = 500 mm, Φ = 400 mm)
• Beam Profile Monitor (BPM)
• Target support controlled by 4 motors
• Fixed (165^o) and mobile (10^o-150^o) charged particle detector
• Electronic modules and power supplies
• Preamplifier
• Spectroscopic amplifier
• Voltage source
• Data acquisition system with MCA Fastcom and MPANT
• NIM rack with power supply
• CCD video camera (x7)
• Liquid nitrogen trap
• Carousel for foils (used at ERDA)
• Retractable SDD detector for PIXE
• HPGe retractable detector, cooled with liquid nitrogen for PIGE
• Beam extraction system in the air, with XYZ automated target support (radiobiology, in-air PIXE)

8. Beam line with reaction chamber for implantation and irradiation with ion beams

• Electrostatic beam sweep system (17x17 cm²)
• BPM monitoring with 4 Faraday cups with secondary electron suppression
• Sample carousel with manual operation
• Sample holder heated up to 800 ^oC
• Sample holder cooled with liquid nitrogen
• Beam stop

9. Reaction chamber beam line for cross-section measurements

• Cylindrical vacuum chamber (h=500 mm, φ=400 mm)
• Fixed target support
• Mobile detector support (10^o-150^o)
• 11 ports
• Collimator system
• 2 Faraday cups

Examples of ionic species^* that can be accelerated at 3 MV Tandetron

Accelerated particle	Energy up to MeV (estimated values)	Beam current up to eµA
¹H	6	30^**
⁴He^***	9	1
⁶Li	12	0.25
⁷Li	12	0.1
¹⁰B	18	5
¹¹B	18	20
¹²C	20	80
¹³C	20	10
¹⁴N	20	0.5
¹⁶O	20	50
¹⁹F	20	0.05
²⁶Mg	20	0.2
²⁷Al	20	1
²⁸Si	25	70
³¹P	25	70
⁵⁸Ni	25	20
⁶³Cu	25	20
⁷⁵As	25	10
¹⁹⁷Au	5	10

The values in the table above for energy and current are maximum values and cannot be met simultaneously.

Note:With he exception of noble gases, all stable elements in the periodic table can be accelerated.

* For more details regarding the beam currents that can be obtained using the 860A source, please see R. Middleton, A Negative Ion Cookbook
** This value of the proton beam current can only be obtained from the duoplasmatron source (358).
**** The alpha particle beam is obtained only from the duoplasmatron source.

References:

[1] I Burducea et al., Nuclear Instruments and Methods in Physics Research B, 2015
[2] C Podaru et al., Nuclear Instruments and Methods in Physics Research B, 2013