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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
Projectile Fragment Expert’s Workshop.
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)
Projectile Fragment Expert’s Workshop.
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
Projectile Fragment Expert’s Workshop.
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
Projectile Fragment Expert’s Workshop.
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
Projectile Fragment Expert’s Workshop.
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
Projectile Fragment Expert’s Workshop.
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
Projectile Fragment Expert’s Workshop.
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
Projectile Fragment Expert’s Workshop.
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
Projectile Fragment Expert’s Workshop.
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
Beam Thermalization: 40S fragment
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
Projectile Fragment Expert’s Workshop.
CS, Sept 01 #11
Beam Thermalization: 40S fragment
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
Projectile Fragment Expert’s Workshop.
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+
Beam Thermalization: 40S fragment
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
Projectile Fragment Expert’s Workshop.
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)
Projectile Fragment Expert’s Workshop.
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
Projectile Fragment Expert’s Workshop.
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)
Projectile Fragment Expert’s Workshop.
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]+
Projectile Fragment Expert’s Workshop.
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
Projectile Fragment Expert’s Workshop.
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
Projectile Fragment Expert’s Workshop.
CS, Sept 01 #19
Wedge for High Intense Beam
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
Projectile Fragment Expert’s Workshop.
CS, Sept 01 #20
Wedge for High Intense Beam
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
Projectile Fragment Expert’s Workshop.
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
Projectile Fragment Expert’s Workshop.
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
Projectile Fragment Expert’s Workshop.
CS, Sept 01 #23
Thank you