Generation of High Intensity Positron Beam Using 20 MeV

linac

Sergey Chemerisov and Charles D. JonahChemistry Division, Argonne National Laboratory

March 25, 2009Jefferson Lab

Newport News, VA

Timeline of the positron source development at ANL

October 2003 ANL was approached about the possibilyty of setting up a positron- production facility at the CSE Division linac

19 and 20 August 2004 Invitational Workshop on Linac-based Positron Beams

September 2004 Memorandum of understanding was sent to LLNL for the loan of of the positron-production equipment.

May 2005 Positron front end arrived from LLNL September 2005 First slow positron beam was

measured at ANL linac February 2006 Improvements to the positron

transport system were implemented. Positron beam with conversion efficiency of 3.5 x 10-8 slow positrons per fast electron was measured

June 2008 new positron converter/moderator assembly was installed and tested

Acknowledgements

Ashok -- Palakkal Asoka-Kumar (formerly LLNL) Hongmin Chen (University of Missouri, Kansas City) Ken Edwards (United States Air Force) Wei Gai (Argonne National Laboratory) Rich Howell (formerly LLNL) Alan Hunt (Idaho State University) Jerry Jean (University of Missouri, Kansas City) Charles Jonah (Argonne National Laboratory) Jidong Long (Argonne National Laboratory) David Schrader (Marquette University) Al Wagner (Argonne National Laboratory) Lawrence Livermore National Laboratory

Funding DOE US Air Force Research Labs

Characteristics of Argonne Linac

L-band 20 MeV no-load energy Steady-state mode 15.5 MeV at 1-amp pulse current Steady-state mode 14 MeV at 2 amp pulse current Peak current at 30-ps pulse of 1000 A Repetition rate 0-60 Hz (can be increased by about a factor of 5) Pulse width 30 ps-5 sec Maximum average current 200 A due to windows thermal load limitations. 1/12 sub harmonic buncher (108 MHz)

Positron Source layout

Penning trap

PALS and DB

PAES

Microprobe

linac

Installed equipment

Existing equipment, not installed

Proposed equipment

Diagram of positron transport

Shield

Converter/moderator

Microchannel plate

Vacuum valve

Up and Down 30 degree solenoid

R 6” Aperture

Leadshield

Radiation Detector

Present condition of positron production line at CSE division linac

Front end

Output end

detector

shielding

bends to separate electrons from positrons

Characteristics of Positron system

First measurements were done using 1-cm thick tungsten target that was borrowed from LLNL -- about a factor of 5 too thick for our energy range

Moderator was either the original vaned LLNL moderator or that supplemented by 3 layers of tungsten mesh

New converter is 2 mm thick. Converter holder is water cooled, but converter itself is not.

New moderator is 10 layers of tungsten mesh Transport system uses 4-inch stainless-steel tubing Positrons are guided using both Helmoltz coils or a solenoid

Band holdingModerator in bright spot

from thick part of mesh

Sharp focus shows little space-charge effect

Signal from microchannel plate detector

Positron(moderator +)

Radiation(moderator -)

Background(beam off)

Na22

0.511 MeV (positron-annihilation

counting

Microchannel plate current as a function of voltage

50 volts22 volts full current22 volts (shortened pulse)10 volts

pulse

The higher the voltage, the sooner the positrons come out

Energy dependence for slow positron production

Difference between experimentally measured positron yield and total number of positrons leaving is due to the difference in the energy spectrum of the positrons

Improvements

New converter and moderator configuration (installed) According to EGS calculation, using a converter optimized for our beam

energy and a repositioned moderator will improve flux by factor of 10. Moderator thickness is not optimal judging from bright spots on the MCP image.

Couple apparatus to linac and remove window limitation Window limits the electron current to 200 A; without window we should be

able to put out 600 A (factor of 3 in positron intensity) Increase linac power by installing new power supplies. That will increase repetition rate from 60 Hz to 300Hz or factor of 5 of the

average current. Use single crystalline moderator in reflection mode. Apply electrostatic potential between converter and moderator (factor of 3). Total improvement is 450 times

New Moderator-converter

Beam stop

e-

e-

e-

New converter/moderator chamber

Existing setup

Table 1 Table 2

e-

e+

e- e+

Positron flux

Technique Positrons per second

Measured with 100A beam,1 Amp peak

1.5 x 107

As is with 200A beam1A peak current 3.0 x 107

Modify converter/moderator

3.0 x 108

Couple directly to linac 6 x 108

Use reflection geometry/ increase linac power

3.0 x 109

How to increase yield of slow positrons?

Increase moderation efficiency

Avoid moderation entirely

It is known that moderation is much more efficient if the positrons are at lower energies. If we can lower the energy of the positrons exiting a converter, we should be able to moderate more efficiently.

If we can bunch the positrons into a narrow energy range, we should be able to inject them into a Penning-type trap and slow them via natural processes

How have we explored these options “in silico”?

Yield of slow positrons as a function of positron energy

The slowing and bunching of positrons

We have used the EGSnrc program to simulate the yields of positrons as a function of energy. We have used the yield of positrons “stopped” (reduced to less than 2 keV) within 1 micron of the surface as a proxy for the yield of slow positrons.

We have simulated an RF cavity, drift space, magnetic fields and phase of RF using the program Parmela.

Positron moderation efficiency calculations

Fast e+

Slow e+ reflection

Slow e+ transmission

W foil

1 m 1 m

50 m

10

10-5

10-4

10-3

10-2

10-1

Fra

ctio

n o

f p

osi

tro

ns

sto

pp

ed

5 67 0.1 2 3 4 5 67 1 2 3 4 5 67 10Energy (MeV)

ReflectionTransmission

Geometry used for positron yield calculationFraction of the positron stopped in 1 m layer of the moderator

Positrons stopped as a function of energy

14

12

10

8

6

4

2

0original reflection

shifted reflection

original transmission

shifted transmission

Yield from shifting spectrum by 100 keV

Yield is relative to transmission = 1

200

150

100

50

0

Po

sitr

on

co

un

t

86420Energy, (MeV)

100 keV shift

Energy spectrum of the positrons produced in 2 mm W target bombarded with 15 MeV electrons

Comparison of the slow positron yield for original and shifted by 100 kev energy distribution for transmission and reflection

Advanced techniques for better positron moderation

E

I

t

E

t

E E

t

Drift positrons to achieve spatial separation

Use RF cavity to “uniformize” the energy of the positrons

Use RF cavity or electrostatic potential for deceleration

Calculations

Schematic of the slow-positron beam-line design, cavity gap=5cm, considering the fringe field, the

total length of affected region along z is set 25cm. In the AMD, magnetic field along z axis decreases from 10000Gauss to 720 gauss from entrance to

exit (100 cm). The field in the AMD satisfies optimized design equation.

(1)z

Bz

129.01

10000

(a) transverse phase ellipse of the beam at the AMD entrance, (b) transverse phase ellipse of the beam at the exit; horizontal coordinator is x axis in cm, vertical coordinator is x prime (Px/Pz) in mrad.

a

b

Compression and translation of positron spectrum

Energy spectrum of the positrons before and after one 108 MHz cavity optimized for the number of positrons in the narrow band (60-80 keV) and wide band (0-100 keV)

0

100

200

300

400

500

600

0.01 0.1 1 10

No.1-Originla Spectrum

No.2- Result from case 1

No.3-Result from case 2

Energy spectrum comparison for cavity that operates at 108 MHz and cavity that have 108 and 216 MHz frequencies. Case 1, the peak value is around 873 positrons out of 59034 within [80keV,100keV] or 1.47% . Case 2, the peak location shifts to [40keV,60keV] while value raised to 1.6% of total positrons.

In both cases, the average axial electrical fields are less than 5MV/m in the cavity.

Increase in yield expected

Where is the “sweet” spot for slow positron production?

Relative yield of positrons as a function of the incident electron energy. The yield of total positrons increases virtually continuously (closed squares) while the number of thermalized positrons appears to approach saturation at about 60 MeV both for reflected moderation (filled circles) or transmitted moderation (open circles). If one is going to design an electron-linac-based positron source the

optimal electron energy for positron generation will be in of 40-60 MeV range.

Summary

We have substantial yield of slow positrons at present (~108 slow e+/s)

Simple techniques to increase the power on the converter target should enable a substantial increase in positron flux

Accelerator-based techniques to alter the energy spectrum of positrons have potential to increase slow positron flux by 2 orders of magnitude.

The ideal accelerator for slow positron production is in 40-60 MeV energy range