Thermalization and Extraction of Projectile Fragments · 2016. 10. 11. · Chandana Sumithrarachchi Outline Introduction to beam thermalization at NSCL Experimental results Challenges

Projectile Fragment Expert’s Workshop.

CS, Sept 01 #1

Thermalization and Extraction of Projectile Fragments

Chandana Sumithrarachchi

Outline

Introduction to beam thermalization at NSCL

Experimental results

Challenges and improvements

Future: Advanced Cryogenics Gas Stopper (ACGS)

Future: Cyclotron Stopper


CS, Sept 01 #2

Why Thermalized beams

Projectile Fragmentation provides wide range of very exotic nuclei at high energies without decay losses and without chemical separation.

Study properties of exotic nuclei far from stability Search for driplines

Some experiments are only possible with low energy beams (0 – 100 keV).

High precision mass measurements Laser spectroscopy (Charge radii, nuclear moments) Nuclear astrophysics experiments (Safe coulomb excitation, transfer

reactions)


CS, Sept 01 #3

Scheme for Thermalization of Projectile Fragmentation

Production of fragments from high energy beam Large momentum spread due to reaction mechanism and production target.

Br and DE separation A1900 spectrometer (High acceptance: 5% Dp/p) , achromatic wedge

Momentum compression and thermalization Narrow momentum spread beams lead to high stopping efficiency (L. Weissman et al.

NIM A 522 (2004) 212)

Gaseous ions collection

Low energy transport

Method for producing an ideal incident beam: Thermalize beam at the object point Bunch momentum spread with wedge at the dispersive

focal plane (H. Weick, et al., NIM B164-5(2000)168; H. Geissel, et al., NIM A282(1989)247)

Cyclotron Separator Gas CatcherRFQ Ion Guide

TargetHigh energy fragment beam Degrader Wedge

+

Thin window

Ions Thermalized ~ eV

80 – 100 MeV/u

++

+++

+

He filled gas cell

RF Fields

DC FieldsGas flow

Low energy beamsTo experiments

Object point Dispersive focal planeDegrader Wedge

Dispersive elements

Accelerate with high voltage


CS, Sept 01 #4

History of Beam Thermalization at NSCL

NSCL has pioneered the beam thermalization in gas since 2002.

Operated 25 L volume gas catcher, 1 bar, 298 K Delivered thermalized beams to LEBIT for Penning trap mass

measurements First high precision mass measurement: 38Ca2+

NSCL expand it’s capabilities to provide thermalized beams to low energy experimental areas (since 2012).

LEBIT

Gas Catcher (2004 – 2009) by D. J. Morrissey


CS, Sept 01 #5

A1900 fragment separator

Low Energy Beam Area at NSCL

Beam thermalization facility (N4 vault)

User Demand: (ReA3 & BECOLA)High rates and high purity

LEBIT (Mass measurements)

BECOLA (Laser Spectroscopy)

EBIT & Reaccelerator

Low energy beam area

User Demand: (LEBIT)Very low rates with high purity


CS, Sept 01 #6

Beam Thermalization Facility

Future: Advanced Cryogenics Gas Stopper (ACGS)

ANL gas catcher

A182

AC206

AC233

RFQ ion guide

North HV

South HV

D0952

D1000 D1030

WedgeDegradersPIN detector

DegradersPIN detector

DegradersPIN detector

Mass Analyzer

Beta detectorFaraday cup

Object pointDispersive focal plane

Beta detectorMCPFaraday cupBeta detector

MCPFaraday cup

Large gas catcher constructed by Argonne National Lab


CS, Sept 01 #7

ANL Gas Catcher

Al thin Window

Nozzle1.3 mm diameter

Large linear gas catcher constructed by Argonne National Lab

120 cm long gas catcher operate at pressure of 70 Torr and temperature of -10 0C

Operate with Radio-frequency (RF) + DC voltage gradient

RF electrodes in Gas Catcher Differential Pumping Station

ANL Gas Catcher

Ion Extraction System

Current measurements capabilities: Window current ( (-) ions) RFQ ion guide electrode current D0952 Faraday Cup Current

D0952


CS, Sept 01 #8

RFQ Ion Guide

RFQ operates at: RF frequency range of 3 -4.5 MHz Peak-to-peak amplitude ~ < 500 V

Beam cooling with He Transverse confinement with RF quadrupole electric field Axial drag field with DC voltage gradient

RFQ electrodes


CS, Sept 01 #9

Beam Thermalization: 40S fragment

40S Fragment Production @ A1900

1175 mg/cm2

Bρ 3,4= 3.1824 Tm

Bρ 1,2= 3.5163 Tm

Wedge 390 mg/cm2

48Ca 140 MeV/u primary beam

Production Rate ~ 5.9E5 ppsIncoming beam energy ~ 75 MeV/u

Purity = 92% dp/p = 2%

40S 41Cl

42Ar

DE

TOF (ns)

PID @ A1900 focal plane

DE

TOF (ns)

40S


CS, Sept 01 #10

0.E+00

2.E-09

4.E-09

6.E-09

8.E-09

1.E-08

1.E-08

2500 2700 2900 3100 3300 3500

Cu

rren

t (A

)

Degrader Thickness (microns)

Positive CurrentScaled

Negative Current


Incoming rate ~ 5.9 E5 ppsIons: 40S, 41Cl &42Ar

40S Fragment thermalized with adjustable degrader and fixed angle wedge at the dispersive focal plane

Object point

Dispersive focal plane

Degrader 2521 mm

Wedge 1016 mm Angle 4.9 mrad

19% of total beam captured in gas catcher

LISE++ simulation

Ions thermalized in 1200 mm long gas catcher

Lost on the back wall

Lost before gas catcher

Range (mm): window projection

40S

40Cl

40Ar

T1/2 = 8.8 sec

T1/2 = 95 sec

Yie

ld

Degrader thickness (um)

2600 2800 3000 3200 34000

2000

4000

6000

Beta decay measurement @ D0952

40S41Cl

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

2650 2850 3050

Dec

ay r

ate

(cp

s)

Degrader Thickness (microns)

41Cl

40S

40S 41Cl

42Ar

PID @ AC233

Ionization Curves

Range Measurement


CS, Sept 01 #11


Incoming rate ~ 5.9 E5 ppsIons: 40S, 41Cl &42Ar

40S Fragment thermalized with: 1) first degrader at the object point 2) second adjustable degrader and fixed angle wedge at the dispersive focal plane

32% of total beam captured in gas catcher

Lost before gas catcher

40S

40Cl

40Ar

T1/2 = 8.8 sec

T1/2 = 95 sec

Object point

Dispersive focal plane

Degrader 1246 mm

Wedge 1016 mm Angle 4.9 mradDegrader 1426 mm

Two degrader sets

Yie

ld

Degrader thickness (um)2600 2800 3000 3200 3400

0

2000

4000

6000

Beta decay measurement @ D0952

LISE++ simulation

Ions thermalized in 1200 mm long gas catcher

Lost on the back wall

Range (mm): window projection

Rate increased by 1.7 times Dispersion match to wedge


CS, Sept 01 #12

35 40 45 50 55 60

0

200

400

600

800

40SO+

Co

un

t ra

te (

cp

s)

Mass (amu)

40S+


Activity mass distribution

20 30 40 50 60 70 80 90

0.0

2.0E-10

4.0E-10

6.0E-10

Ar+ N

+

4

N+

2

FC

_D

10

30 C

urr

ent (A

)

Mass (amu)

O+

2

Stable ion mass distribution

Processes inside the gas catcher:

Thermalization process produces ions and electron pairs. ( )

Form stable molecular ions from impurity molecules in the gas.

Transport of thermalized ions in the buffer gas is affected by the interactions with molecular ions in the gas. (Drift time ~ 70 ms, Depends on impurity concentration, fragment

chemistry)

He2+

Transport through low energy beamline

Mass distribution of 40S Fragment after thermalization


CS, Sept 01 #13

Thermalization and Extraction Efficiency

Measurements were made between AC233 and D0952 detectors. (corrected for beam currents and decay.)

Total Efficiency Includes stopping and extraction efficiencies of the gas catcher, and RFQ efficiency.

RFQ extraction efficiency ~ 80 % Incoming particle rate to the gas catcher varies from 102 to 108 pps.

Total Efficiency Measurement

PIN detector

Beta decay detector

Efficiency Measurement

# of ion pairs * Incoming beam rate Stopping volume Q – Ionization rate density =

1.0x107

1.0x108

1.0x109

1.0x1010

0.02

0.1

1

47

K

37

K 46

Ar

40

S

To

tal E

ffic

iency

Q (Ion Pairs/ cm3/s)


CS, Sept 01 #14

Challenges in Beam Thermalization Process

Total efficiency of thermalization and extraction reduces with incoming beam rate and start saturation. Space charge build up

Nozzle size ( diameter ~1.3 mm) Higher DC gradient Fast extraction methods (Ion surfing on RF carpet –

Future ACGS, Cyclotron Stopper)

Chemical adducts formation with fragments

Stable ion contaminants

Decay product as contaminants

Total Efficiency Measurement

# of ion pairs * Incoming beam rate Stopping volume Q – Ionization rate density =

1.0x107

1.0x108

1.0x109

1.0x1010

0.02

0.1

1

47

K

37

K 46

Ar

40

S

To

tal E

ffic

iency

Q (Ion Pairs/ cm3/s)

Saturation start at ~ 7E106 pps


CS, Sept 01 #15

Chemical Adducts Formation

Install a pump directly attached to the gas catcher (Clean up purpose)

20 30 40 50 60 70 801

10

100

1000

10

100

1000

10

100

1000

10

100

1000

37K

1+

-100 V

0 V

-75 V

Count ra

te (

cps)

Mass/Charge

-50 V37

K2+ 37K

1+

37K

1+

37K

2+

37K

1+37K2+

[37

K(H2O)

3]2+

[37

K(H2O)

5]2+

2+ molecular ions breaks and 37K2+ produces when offset voltage increases.

37K Molecular Breaking

Impurity molecules in buffer gas form molecular ions with fragments (Depends on impurity concentration & fragment chemistry).

Reduce thermalized beam rate for low energy experiments

Mass distribution of 37 K (before clean up)

Apply additional voltage at RFQ

Mass distribution of 37K (after clean up)


CS, Sept 01 #16

[H5O2]+

[H7O3]+

[H9O4]+

[H11O5]+

[CH3(H2O)2]+

Stable Ion Contaminants

MCP image @ D1030 (set for mass = 29)

Stable ions mass distribution (before clean up)

Activity identify from beta detector and slits

Mass analyzer resolution (R) m/Dm ~ 1500

Some cases, stable ions can be rejected with slits

Stable ion mass distribution (after clean up)

Stable ions are formed during thermalization process.

Contribute to high beam current(~ 600-800 pA)

Major issue for low rate experiments Activity on the left

Stable and radioactive masses at A=29e.g. COH+

This is all A=29

37K experiment

29P experiment : Mass measurements

CO groups show: gas catcher become very clean

34Ar experiment

20 25 30 35 40 45 50 55 60 65 70 75 801

10

100

1000

10000

[34

ArCO]+

[34

ArCO]2+C

ou

nt

rate

(cps)

Mass/charge

[34

Ar]+


CS, Sept 01 #17

Decay Products as Contaminants

ReA3 Experiment: Measurement of the 75Ga(a,n) and (a,2n) cross sections important for neutrino driven wind nucleosynthesis – Z. Meisel

75Ga beam was thermalized in ANL gas catcher and delivered to reaccelerator.

Stable ion contaminants were expected from EBIT, BCB and gas catcher.

Beam is 95% pure 75Ga and found 1.1% of 75Ge (daughter of 75Ga)

Decay products come from gas catcher

75Ga

75Ge

75As

T1/2 = 126 sec

T1/2 = 82.8 min

Stable

Thermalized beams are some time contaminated with decay products.

75Ge

ReA3 Experiment: Measurement of the 34Ar(a,p)37K reaction cross section – K. Chipps

34Ar beam was delivered for reacceleration

Beam composition: 34Ar (38%), 34Cl (16%), 34S (46%)

Decay products come from gas catcher

34Ar

34Cl

34S

T1/2 = 843 ms

T1/2 = 1.53 sec

Stable


CS, Sept 01 #18

Wedge for High Intense Beam

Homogeneous wedges can be manufactured with glass

High intense beams can damage fused silica glass wedges

Damage glass wedge


CS, Sept 01 #19


In-house built Al wedge ( size: 15 cm X 15 cm; angle = 5 mrad; middle thickness = 1.0 mm)

Check the wedge for homogeneity with 82Se beam

Beam position on the wedgeThicker Thinner


CS, Sept 01 #20


0.00E+00

1.00E-01

2.00E-01

3.00E-01

4.00E-01

5.00E-01

6.00E-01

7.00E-01

8.00E-01

1250 1450 1650 1850 2050

Posi

tive

cu

rren

t (n

A)

Thickness (um)

B- Strip (Positive)

1b I+

2b I+

3b I+

4b I+

5b I+

6b I+

7b I+

8b I+

0.00E+00

1.00E-01

2.00E-01

3.00E-01

4.00E-01

5.00E-01

6.00E-01

7.00E-01

8.00E-01

1250 1450 1650 1850 2050

Posi

tive

cu

rren

t (n

A)

Thickness (um)

A- Strip (Positive)

1a I+

2a I+

3a I+

4a I+

5a I+

6a I+

7a I+

8a I+

0.00E+00

1.00E-01

2.00E-01

3.00E-01

4.00E-01

5.00E-01

6.00E-01

7.00E-01

8.00E-01

1250 1450 1650 1850 2050

Posi

tive

cu

rren

t (n

A)

Thickness (um)

C- Strip (Positive)

1c I+

2c I+

3c I+

4a I+

5c I+

6c I+

7c I+

8c I+

C-Strip

B-Strip

A-Strip

1 2 3 4 5 6 7 8

Thicker Thinner


CS, Sept 01 #21

Improvement: Tunable Wedge System

Tunable wedge system installed recently.

Two fused silica wedges rotate opposite direction to get the desired angle

Angle per wedge = 2.5 mrad; middle thickness = 0.5 mm; Max wedge angle = 5 mrad

Tested with 75Ga beam

0

1

2

3

4

5

6

7

8

1620 1670 1720 1770 1820 1870

Neg

ativ

e C

urr

ent

(nA

)

Degrader Thickness (mm)

Ang = 0 mrad

Ang = 0.6 mrad

Ang = 1.7 mrad

Ang = 3.3 mrad

Ang = 5.0 mrad

Degrader 355 mm

Tunable wedgeMax. ang 5 mrad

Degrader1250 mm

Dispersive elements

Preliminary results

Stopping efficiency can be increased by having tunable wedge system. 75Ga experiment


CS, Sept 01 #22

Outlook

Beam thermalization facility at NSCL provides beams successfully to low energy experimental programs

Momentum compression improves beam thermalizationefficiency

Challengers for beam thermalization were identified and some of improvements were implemented

New beam thermalization capabilities are on the way to reality soon (ACGS, Cyclotron Stopper)

• First Beam to ANL gas catcher : Aug 2012

ANL Gas Catcher Operation

• Beams for LEBIT

• Fe-62,63,67

• Co-63,64,65,68,69

• Br-72

• O-14

• N-13

• C-11

• Cl-31

• Si-24

• P-29

• Na-21

• Beams for BECOLA

• Fe-51,52,53

• K-35,36,37

• Beams for ReA3

• Ga-76

• K-37

• Ar-46

• K-46

• Ar-34

• Ga-75

• Gas Catcher experiments

• Ga-76

• K-37,38,47

• P-29

• Cl-33

• S-40

• Mg-29• O-14• Si-26• Br-72• Kr-73• Ar-46


CS, Sept 01 #23

Thank you

Documents

Thermalization and Extraction of Projectile Fragments · 2016. 10. 11. · Chandana Sumithrarachchi Outline Introduction to beam thermalization at NSCL Experimental results Challenges