Study on the Injection System for Compact Cyclotron Mass Sp - PowerPoint Presentation

Slide1

Study on the Injection System for Compact Cyclotron Mass Spectrometry

Do

Gyun

Kim

,

Joonyeon

Kim, H. C. Bhang

Department of Physics, Seoul National University

C. C.

Yun

Department of Physics,

Chu

ng-Ang

University

Jong

-Won Kim

National Cancer

CenterSlide2

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 dating14C is needed to separate from the 14N, 13CH and 12CH2Mass 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)

한국지질자원연구원

1

MV AMS

장비

( 2007) Slide3

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

MeV

Size: ~ 7x5m

2

(for NEC 500kV tandem AMS system)

Difficult in maintenance (ex. SF6 insulating gas)Slide4

Cyclotron AMS

Schematic configurations of the cyclotron AMS system.

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 sample

blank

Mass resolving power (m/

Δ

m): ~1900 (14C&13CH)

Beam energy: ~100keV

Size: ~ 3x2.5m

2 Slide5

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 Slide6

Design of Cyclotron AMS

We 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. Slide7

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 spacesSlide8

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 TRANSPORTBeam separation at the focal point F2 simulated using TURTLE.Slide9

Ion Source Assembly

The 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.Slide10

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

Slide11

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.Slide12

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.Slide13

Design of Dipole Magnet

A 90



dipole magnet has been designed using RADIA software. Bend radius: 30cmEdge angel: 30°Pole gap: 45mmMagnetic rigidity (14C, 30keV): ~0.1 Tm Required magnetic field: ~3.1 kGTotal current : ~6000 A Bump size (Bump height: 0.5mm) Inner bump: 9mm, Outer bump: 15mmGood field region(ΔB/B<0.1%):

~4 cmSlide14

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.Slide15

감사합니다.