TRIUMF Centre for
Molecular & Materials Science
8 Year Plan 2007-2015
Jess H. Brewer - 6 Dec. 2006

Proposal:

Design Study for a Surface Muon Source

in the present Proton Hall (2010-2015)

A Five-Year Plan should include the following categories:

Maintenance & Operation of existing facilities and programs.

Construction & Commissioning of finished engineering designs.

Engineering Design of thoroughly evaluated new concepts.

Concept Evaluation: Comparison of scientific potential, technical feasibility and probable cost of competing proposals.

Guiding Principles:

Do what you're good at.

Go for the Gold.

Recapitulating the Obvious:

What We're Good At

Making Muons - e.g.

Surface µ+ beam invented by U. Arizona group at LBL but developed at TRIUMF. Now ubiquitous and indispensable at all muon facilities.

Ultra low energy µ+ beam invented at TRIUMF but developed at PSI because of rates. Now world's most oversubscribed muon channel.

Using Muons - e.g.

Spin Rotators developed & perfected at TRIUMF.

RF-µSR spin echo first achieved at TRIUMF.

Surface Muons

500 MeVProton Beam

π+ : all energies & angles.

µ+: 4 MeV,

100% spin polarized

Some pions stop in “skin” of

production target and decay.

Range: 150±30 mg/cm2.

Bright, imageable source.

T1

New µE4

World's only operating

ultra-Low Energy Muon

beam facility at PSI:

~103 µ+/sec

(vs. TRIUMF's 8Li β-NMR)

µ+SR

expt.

History of

µ+ stopping luminosity:

Enabling µ+SR

One possible design:
Leaky Magnetic Bottle

Reflection criterion:

Bmax~ 0.6 T

B0~ 0.2 T

 crit ~ 35o ⇒  escape ~ 1.5 sr

Place production target in a field between two rad-hard coils (proton beam into page).

Low energy pions return to skin of production target (textured to make every surface both an entrance and an exit surface).

Surface muons escape if  0 <  crit

(equivalent to an acceptance of

1/4 of entire 2π solid angle).

Compare  ≈ 50 msr for typical surface muon channel: factor of

30

from solid angle alone.

Cyclotron Vault

EXISTING PROTON HALL

150 µA beam dump

(and UCN source?)

8Li Source ?

Surface Muon Source

Custom Production Target

inside a “magnetic bottle” for pions

“Dai Omega” style

High Flux Surface µ+ Beam

Ultra Low Energy

µ+ Beam

Up to

150 µA

500 MeV

BL4S

β-NMR

Kickers and splitters

to multiple areas

Grade Level

Grade Level

Up to

250 µA

450 MeV

BL4N

to ISAC+

Proposed

ultra-Low Energy Muon

beam facility at TRIUMF:

~104 µ+/sec

OR . . . re-accelerate

to ~ 500 keV and

focus on very small spot.

“Leaky Magnetic Bottle”

Bent

Solenoid

GRADE

LEVEL

New Science Opportunities

Simply increasing Low Energy Muon intensity from 103 to 104 µ+/s is a huge step for LE-µSR.

Combined with β-NMR, probe thin films, multilayers, magnetic nanostructures, . . . .

Muonium in gases; hydrogen isotope chemistry.

Re-accelerate LEM to ~ 1 MeV  parallel beam can be focused onto µm-sized spot:

“Scanning µSR Microscope”?

Schedule & “Bare Minimum” Costs

Working Backward:

2015: Start Construction

2014: Finalize details

2013: Next 5YP firm

2012: Converge

2011: Choose winners

2010: Develop designs

2008: Recruit people

People Costs:

Beam Optician $75K/y

Engineer $75K/y

Technician $50K/y

Other Costs:

Prototypes $300K

Test Expts $200K

TOTAL $ 1.5 M (2010-15)

The End

“Appendices” follow . . .

Cyclotron Vault

EXISTING PROTON HALL

150 µA beam dump

BL5

Surface Muon Source

Custom Production Target

in a “magnetic bottle” for pions

“Dai Omega” style

High Flux Surface µ+ Beam

Low Energy µ+ Beam

150 µA

BL4

150 µA

(Reroute

to ISAC)

Or extract these beams

simultaneously from the

BL4 port and bring a

beam for the UCN

source in from BL5.

Details of Production Target:
Magnetic Bottle for +

Low energy pions return to “fin” of production target (every surface is both an entrance and an exit surface).

At each pass, the pion loses energy and is scattered.

The + spectrum is almost flat near zero energy, so each pass through the “fin” is another chance to stop within the “muon skin”. But at each pass, a fraction f of the pions “leak out of the bottle”. Thus the “surface enhancement factor” relative to a plain target is

ε = ∑ (1 – f )n = 1/f

+

n=0

∞

For f = 1.5/2,

ε  4.2

+

Production target thermally coupled to thin metal disk (“fin”) viewed here “edge on”.

EXISTS!

 = 1.3 sr!

(ADD 5 YEARS)
Proton  Muon Hall: Critical Path

2003

2005

2007

2009

2011

2010

2012

High Time Resolution

Transverse Field (TF)-µSR

Typical time spectrum (histogram)

CW only

CW (PSI & TRIUMF) vs. Pulsed (ISIS, J-PARC) Muon Facilities

Time resolution of CW-µSR two orders of magnitude better!

“Themes” in µSR

Muonium as light Hydrogen

(Mu = µ+e-) (H = p+e-)

Mu vs. H atom Chemistry:

- gases, liquids & solids

- Best test of reaction rate theories.

- Study “unobservable” H atom rxns.

- Discover new radical species.

Mu vs. H in Semiconductors:

- Until recently, µ +SR → only data on

metastable H states in semiconductors!

The Muon as a Probe

Quantum Diffusion: µ + in metals (compare H+); Mu in nonmetals (compare H).

Probing Magnetism: unequalled sensitivity

- Local fields: electronic structure; ordering

- Dynamics: electronic, nuclear spins

Probing Superconductivity: (esp. HTcSC)

- Coexistence of SC & Magnetism

- Magnetic Penetration Depth λ

- Coherence Length ξ

AFTERWORD
Town Meeting 2002

Ideally, a 5-Year Plan should include the following categories:

Maintenance & Operation of existing facilities and programs.

Construction & Commissioning of completed engineering designs.

Engineering Design of new facilities chosen by thorough evaluations.

Concept Evaluation: Comparison of scientific potential, technical feasibility and probable cost of competing proposals for new facilities.

Traditionally the final category has been relegated to semi-democratic processes such as TUG AGMs, Town Meetings and the prior efforts of self-organized groups of Users. While new initiatives must always have “grass roots” origins, these partisan efforts must evaluated and compared much more thoroughly than is possible in a year or two before each 5-Year Plan. A step is missing.

I therefore proposed (in 2002) that TRIUMF create a standing LRPC to fulfill this role. This body would receive proposals asynchronously and review them full time, thus alleviating the “Communication Bottleneck” .

2005 : Communication Task Group recommends creative use of Web-based tools (databases, interactive forms, wikis etc.) to facilitate User INPUT.

Jess Brewer

2002-09-19T16:06:31

Jess H. Brewer