Study on the Injection System for Compact Cyclotron Mass Spectrometry

Do Gyun Kim, Joonyeon Kim, H. C. BhangDepartment of Physics, Seoul National University

C. C. YunDepartment of Physics, Chung-Ang University

Jong-Won KimNational Cancer Center

Accelerator Mass Spectrometry AMS is an ultra-sensitive technique for isotopic analysis.

For the last few decays, AMS technique is widely used for radiocarbon dating, environment research, biology, and so on.

Radiocarbon dating• 14C is needed to separate from the 14N, 13CH and 12CH2

• Mass resolving power of a cyclotron = M/ΔM 83,000 between 14C and 14N

1,900 between 14C and 13CH

서울대학교 기초과학공동기기원 3 MV AMS 장비 ( 1999)

한국지질자원연구원1 MV AMS 장비 ( 2007)

Tandem AMS

Negative ion source: prohibit 14N (negative electron affinity) Bending magnet: select carbon isotope and their molecular isobarsNegative ion acceleration: up to terminal voltageStripper stage: change the charge state into positive suppress the molecular isobarsPositive ion acceleration: up to terminal voltage* electric chargeIon selection: 14C – count the particle (SSB detector) 12C & 13C – reading the current

Schematic configurations of the Tandem AMS system.

Beam energy: several MeVSize: ~ 7x5m2 (for NEC 500kV tandem AMS system)Difficult in maintenance (ex. SF6 insulating gas)

Cyclotron AMS

Schematic configurations of the cyclotron AMS sys-tem.

Negative or Positive ion source: Negative ion source for carbon datingRf buncher: focusing the beam in the longitudinal direction Bending magnet & slit system select carbon isotope for alternate accelerationQuadrupole triplet: matching the transverse phase spaceCyclrotron : alternate acceleration of 12C, 13C and 14CIon selection : Rf frequency response curve (using MCP)

14 C sam-ple blankMass resolving power (m/Δm): ~1900 (14C&13CH)

Beam energy: ~100keVSize: ~ 3x2.5m2

Previous Cyclotron AMS

1. LBL(Lawrence Berkeley Laboratory) Cyclotrino: first cyclotron AMS• Very low transmission efficiency: 5x10-5 (0.2 counts/min for a source current of 10 μA)

2. SMCAMS (Shanghai mini cyclotron AMS): recent cyclotron AMS• Transmission efficiency: ~0.1 (25 cps for a source current of 40~50 μA)• Unstable cyclotron magnet system

Design of Cyclotron AMSWe plan to adopt RF buncher and flat-topping RF system to improve transmission efficiency

• Flat-topped RF wave by the third harmonic frequency added Beam phase acceptance: ~ 30°

• A beam is bunched at the injection line by RF buncher • The sawtooth RF buncher we chose is similar to the one built at GANIL* • The RF frequency of the AMS cyclotron : ~0.5 MHz • The number of harmonics considered is around 20. • The frequency of the buncher is in the range of 10 MHz.

* A. Chabert et al., Nucl. Instr. and Meth. A 423 (1999) 7.

Design of Injection Beam Line

RF buncher • It is loacated rather in the upstream (F1) of the beam

line to reduce the required RF voltage. 90° dipole magnet • Bending radius: 30 cm • Edge angles: ~30° (vertical focusing) Slit system • Pre-selection of the isotopes of carbon Quadrupole triplet • Electrostatic or magnetic • Matching the transverse phase spaces

Design of Injection Beam Line

The design of an injection beam line, which extends from the extraction of ion source to the injection at the cyclotron, was carried out using TRANSPORT and TURTLE program

Transverse envelopes of a 14C beam simulated using TRANSPORT

Beam separation at the focal point F2 simu-lated using TURTLE.

Ion Source AssemblyThe ion source assembly mainly consists of filament, anode, extraction electrode, and gas inlet. The extraction system is designed to extract ions with extraction voltage up to 30kV. Aperture diameters of anode and extraction electrode is 0.5mm and 3mm. The extraction electrode is designed to be replaceable for adjustment the gap distance. The front part of the einzel lens is designed to accommodate the extraction electrode.

Beam Extraction Test

Carbon beam extraction test (Co2 gas), Digital camera image Extraction voltage: 15kV, Beam size: 4*4mm2 (@ Einzel lens voltage: 11.5kV)

Einzel lens voltage: 11.0kV Einzel lens voltage: 11.5kV Einzel lens voltage: 12.0kV

Rf Buncher System Sawtooth Rf driver • A triode tube is used as switching device with an input of duty variable square pulses

-Duty variable Pulse signal is fed to Triac.-Buncher is a kind of capacitor, so the electric potential of inner cylinder oscillate like as saw-tooth wave.

Rf Buncher System

Buncher is located right after the Einzel lens. This location allows use of a lower voltage and a longer drift space. This can reduce beam-energy spreads, but the coupling of rf bunching effect with the dipole bending seems to make the width of beam phase bunched at the injection point difficult to be controlled.

Beam energy spreads versus the distance of drift space for two different rf voltages.

Motions of 6D phase spaces and the beam envelopes calculated using TRACE3D.

Design of Dipole Magnet

A 90 dipole magnet has been designed us-ing RADIA software.

Bend radius: 30cm Edge angel: 30° Pole gap: 45mm Magnetic rigidity (14C, 30keV): ~0.1 Tm Required magnetic field: ~3.1 kG Total current : ~6000 A Bump size (Bump height: 0.5mm) Inner bump: 9mm, Outer bump: 15mm Good field region(ΔB/B<0.1%): ~4 cm

Conclusions

Beam extraction experiments was performed using CO2 gas. 90 dipole magnet will soon be constructed (next month). Rf buncher is manufactured. But the sawtooth RF driver will be

modified (10MHz). Comparison of the beam optics calculations with the beam

measurements will help in better matching the beam phase space to cyclotron acceptance.

We expect that optimal injection line design will be revealed by this work.

감사합니다 .