Laser Spectroscopy

Lecture 2

Bradley Cheal, University of Liverpool

Properties from optical spectra

• Isotope shifts

➔ Charge radius

• Hyperfine splitting

➔ Nuclear spin (measurement)➔ Magnetic dipole moment➔ Electric quadrupole moment

�hr2i ! �h�22i

Qs ! h�2i

➔ Sensitive probe of nuclear wave functions

➔ Single particle level migration

➔ Existence of a nuclear state at all

Exploring the nuclear chart with lasers

Where are the magic numbers? Do they change away from stability?

What are the origins andfeatures of collectivityand deformation?

What can single-particle phenomena tellus about the nuclear force?

Do proton emitting nuclei have very large charge radii?

Can we learn about super heavy elements / island of stability?

Why do some nuclei have a halo character?

What are the consequences of pairing?

Obtaining reaction products from targets

But sometimes we want to extract the products in the form of a beam

Measure nuclear decaysin the vicinity of the nuclear reaction

Two beam production methods: In-flight and ISOLIn-flight

ISOL (Isotope Separation On-Line)

Separator

(eg. FRS)Primary beam

Primary beam

Foil targetHigh energyion beam (MeV)

Low energyion beam (~30keV)

Stopping volume Either:

— the volume of a “thick” target itself— or a separate “stopper” - eg. buffer gas

Extraction

Mass separator

Example ISOL facilities for high resolution laser spectroscopy

IGISOL, JYFL, Finland

ISOLDE, CERN

ISAC-I, TRIUMF, Canada

ISOLDE, CERN

Produce singly charged beamsof radioactive isotopes (RIBs)with an energy of eg. 30-60keV

ISOLDE: Isotope Separator On-Line (ISOL) DEvice

Target

Ioniser

Extraction as a beam

30kV

(singly-charged + ions)

ISOLDE: Isotope Separator On-Line (ISOL) DEvice

CERN Linear Accelerators

Part a linear accelerator under construction at CERN, “LINAC 4”

LINAC4 is 80m long and located 12m undergroundAccelerates protons to 160 MeVProduction of proton pulses/bunches

Linear Accelerators

length of nth drift tube / velocity / penergy /

pn

CERN Synchrotrons

Proton Synchrotron Booster (PSB)Takes the beam from LINAC 2/4Accelerates protons to 1.4 GeV~2uA to ISOLDE (~half)

Beam manipulation

High energy beams aresteered and focussed with magnets

Quadrupole lenses

(large) bending magnet

Ion beams at ISOLDE

ISOL beam manipulationDouble x-y steerer

Quadrupole lens (triplet)

Einzel lens

Beam diagnosticsFaraday cup

Currents down to ~1pACan be segmented for position info.

Micro-channel plates, Ion counting with rates < 105/s

Wire grid / beam scanner

Collinear laser spectroscopy

E =1

2mv2 =) �E = mv�v

Collinear laser spectroscopy

Ion Source (30kV)LaserPMT Tuning potential

From ionsource Doppler

broadening

Effects of energy spread and emittance

Wide ion beam(requires wide laser beam)

➔ Residual broadening of spectral peaks➔ Reduction in resolution & sensitivity

➔ Needs higher laser power➔ Increases background

Focussing

➔ Peak skewing➔ Reduction in sensitivity

Problems solved using a cooler…

Ion beam cooler for cooling

• Quadrupole rods with RF appliedfocus the ions to the axis

• Weak axial field guides ions to end

Ions lose energy (and therefore energy spread) through collisions

He buffer gas

Need to reduce the photon background

Ion beam Laser beam

Particledetectors

Segmented photomultiplier tube

Imaging optics

+

-(continuous)

Problem: continuous non-resonant scattering of photons into PMT

Solution: detect photons only in coincidence with ions

… but isobaric contaminants still reduce the effectiveness

Ion beam cooler for bunching

200ms PMT

Apply a trapping potential to the end electrode

Cooler bunching technique

Ungated

Gated (64μs - 70μs)

Time of flight(50ms accumulation)

Background suppression50ms / 6μs = ~104

TRIUMF (UBC, Vancouver)

Accelerator types: Cyclotron

qvB =mv2

r

! =v

r

!c =qB

m

⇒ frequency is constant

⇒ apply via RF to “dees”

TRIUMF Accelerator: Cyclotron

p at 500MeVand up to 100uA

Availability from conventional ISOL facilities

ISOLDE: Thick target, hot cavity

1.4 GeV protons, 2μA

High yields......if chemistry and τ1/2 permit

JYFL, Finland

A complementary technique: IGISOL (JYFL)

• Reaction products recoil from thin foil targets• Slowed or “stopped” in He buffer gas• Products carried out in supersonic jet• Ions captured by fields, gas pumped away

A complementary technique: IGISOL (JYFL)

Cyclotron beam

Thin foil targets

Gas volume

He

JYFL Accelerator: Cyclotrons

MCC30/15p, dBeam time!

K130p, d, α, 32S…

JYFL, Finland

Thin foil targets, He buffer gas,Supersonic gas jet extraction.

• Fast (sub-ms) extraction• Chemically unselective

Beam fromK130 cyclotron(inc. heavy ions)

100μA p @ 30MeV50μA d @ 15 MeV… and n converter

Ions LaserPMT

Magnet

Commissioning of the IGISOL 4 laboratory

Commissioning of the IGISOL 4 laboratory

Tuneable (dye) laser

Leads to a continuous rangeof fluorescence wavelengthsfrom the band head

Complicated molecules with many rotational andvibrational states

Tuneable (dye) laser

Tuneable (dye) laser

CW and pulsed lasers

Commissioning of the IGISOL 4 laboratory

Pulsed laser

In-c

oole

r

Col

linea

r

• Focus of slow / trapped ions ➜ always efficient• Can use broadband/pulsed lasers ➜ large λ range

J=0

J=1

J=1

J=2

Weak?Short λ?

Optical manipulation in the ion cooler-buncher

Cheal et al. Phys. Rev. Lett. 102, 222501

Collinear laser line

Penning trap mass spectrometer RF cooler-buncher

Electrostatic switchyard

Optical pumping at IGISOL 4

Quadrupole moments of manganese

{CECentry

6D,8P,4D...

Atomicground state

No sensitivity toquadrupole moments

Can’t compare shellmodel interactions

Optical pumping in ISCOOL

Optical pumping

A~106/s80% branch

A~2×108/s

C. Babcock, PhD Thesis, University of Liverpool (2016)

Quadrupole moments of manganese

GXPF1A uses full pf spaceLNPS adds the νg9/2 and νd5/2 orbitals

C. Babcock, H. Heylen et al. PLB 760 387 (2016)

ISOL target and ion source (eg. ISOLDE)

In beam is then mass filtereddownstream

Ionisation takes placeusing e.g. surface ionisation

Spectroscopy in the ion sourceCan’t detect photons, so use many lasers to resonantly ionise

Ion detection ordecay spectroscopy

Ionisationpotential

Atomic gs.Tune/scan first step

Ion

coun

tsfrequency

Advantages… and disadvantages59Cu 1/2-1/2 58Cu 1/2-1/2

59Cu 1/2-3/2(HR technique)

(In-Source)

(In-Source)

• Sensitive particle detection(rather than photon detection)

• Doppler broadening• High power lasers - broadband

⇒ Low resolution

(more tolerable if heavy element)

Other Approaches

Collinear Resonance Ionisation

Multiple photon detection

S. Malbrunot-Ettenauer CERN-INTC-I197 (2017)

cf. TJ Procter JPCS 381 012070 (2012)

Extract beams for high resolution spectroscopy

Transport to experiments,including a set-up forhigh resolution laser spectroscopy

(see in a moment)

Problems of isobaric components(for any experiment)

- swamp the signal - misidentification / interpretation Neutron converter…?

Release curve…?

Apply laser ionisation: Laser Ion Source

Deliver to experimentsinc. HR laser spec.

• Try to suppress surface ionisation• Selectively enhance yield

• Purified beam for single Z as well as A

• Higher yield (A,Z)• Lower background

Apply laser ionisation: Laser Ion Source

Post accelerated Zn beam(but isobars still present)

Laser identifies the peakscaused by the zinc

Summary: Laser spectroscopy at RIB facilities

(a) (b)

V=Vdc+Vrf cos(�t)

V=Vdc-Vrf cos(�t)

Ion source

Ion beam Laser beam

Photondetector

Doppler tuningelectrodes

Collinear spectroscopy(high resolution)

or

Cooler-buncher

Particle or decay countingeg. ISOLDE Decay Station

In-source method(higher sensitivity, lower resolution)

Laser beams step-wise resonantly ionize the reaction products leaving the target

(laser on/off comparisons and scanning)

Radioactive isotopes extracted as an ion beam

Mass analyzingmagnet

Cheal, Cocolios and Fritzsche, Phys. Rev. A 86, 042501

IP

gs

photon

ion

ΔE=mvΔv