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The Recoil Mass Separator Project at Notre Dame
Larry Lammfor the Notre Dame RMS Design Team
SNEAP 2008
Outline
• Motivation – why do we need a recoil mass separator?
• Basic Design• Key Elements and Design Strategy• Status of the project• Expected timeline
This blast wave originated in the Cygnus Loop supernova, which occurred about 15,000 years ago.
“The surface of the Earth is the shore of the cosmic ocean. On this shore, we’ve learned most of what we know. Recently, we’ve waded a little way out, maybe ankle deep, and the water seems inviting. Some part of our being knows this is where we came from. We long to return. And we can, because the Cosmos is also within us. We’re made of star stuff. We are a way for the Cosmos to know itself.”
Carl Sagan, “Cosmos: A Personal Journey”, PBS TV, 1980
Nucleosynthesis
1957 Burbidge, Burbidge, Fowler, Hoyle
From H. Schatz, Nuclear Astrophysics course material, jinaweb.org
Radiative Capture Reactions
• Most stages of stellar evolution involve the radiative capture of protons and α particles, and many of the pertinent (p,γ) and (α,γ) reactions have been extensively studied by γ ray detection. However this method is limited at lower energies by the cosmic and the beam induced background.
Stellar Temperatures and Available Energy
• Solar Temp – surface ~ 6000 K• – core ~ 15 x 106 K (T6 = 15)• Maxwell-Boltzman distribution, with energy
peaked around 3/2 kT, where k= 8.62 x 10-5 ev/K
• Core energy ~ 2 keV• Only p-p chain reactions• VERY LOW energy by laboratory
standards
Stellar Temperatures and Available Energy
• But many other stellar scenarios to consider
• Novae, Supernovae – much hotter• Energies of hundreds of keV to a few MeV• Still low by laboratory standards
S(E) extrapolation for the 6.79 MeV excited state in the 14N(p,γ)15O reaction
S FactorLimited by cosmic and
beam induced
backgrounds
S(E) for the 6.79 MeV excited state in the 14N(p,γ)15O reaction
Reaction rate differs from extrapolation by 2 orders of magnitude at certain energies!!!
Must measure as low as possible!
S Factor
Recoil Mass Separator• One way to try to measure at very low energies
is to reduce the inherent background by detecting the γ rays in coincidence with the reaction products, but this necessitates separating the recoils from the beam using a recoil mass separator (RMS) in inverse kinematics. This is an incredibly difficult task, since for typical reactions, the ratio of beam to reaction products will be on the order of 1017. However, with proper design, an instrument capable of this level of separation can be built.
How much is a part in 1017?
• Lake Michigan – 5 x 1015 liters• 50 ml out of Lake Michigan!
Heavy water in Lake Michigan?
122 x 106 liters !!!
(5 x 106 liters of D217O) !!!
Recoil separators for radiative capture studies
• DRAGON, ERNA:– Two very successful separators designed for specific
benchmark reactions• 15O(α,γ)19Ne for DRAGON• 12C(α,γ)16O for ERNA
• ND design:– Broad range of reactions: A<40– Large acceptance: ∆θ<±40 mrad, ∆E<±7.5%
Notre Dame recoil separator: Design parameters
9 mrad0.97 deg.
1.8 %1.25 MeV12.5 MeV
36Ar(α,γ)40Ca
32 mrad1.8 deg.
6.5 %460 keV3. MeV
22Ne(α,γ)26Mg
40 mrad2.3 deg.
7.4%360 keV2. MeV
18O(α,γ)22Ne
∆θ (mrad)(deg)
∆E/E (%)ECM Ebeam
Reaction
Stable beam from the KN (4MV) Van de Graaff acceleratorBeam intensity up to 100 µA (~1015 pps)
Beam mass < ~40 Acceptance
Sample of the list of reactions
St. George
Exis
iting
bea
m li
ne
Charge Selection
Mass Selection
Detection - ID
Jet gas target
KN accelerator
STrong Gradient Electro-magnetic Online Recoil separator for capture Gamma ray Experiments
Charge selection
• Multiple charge state after gas target– Selection of the most
abundant one (~40%)– RMS total efficiency is
charge state dependant– Clean rejection of the other
beam/recoil charge state• Charge selection can be a
source of background:– ∆Q/Q0 can be large
→ selection in two steps
26°
26°
Recoil and beam profile
24Mg(α,γ)28Si5+ @ 8 MeV
1+2+3+4+5+6+7+8+9+10+11+12+
Mass selection: Wien filter
• Crossed magnetic and electric field
• Particle with velocity = E/Bare not deflected
• Choice for flexibility
• Fields homogeneity andE/B constant are criticalfor mass selection
Detector development
Ion optic example
18O(α,γ)22Ne @ 1.94 MeV
24Mg(α,γ)28Si5+ @ 8 MeV
Magnet status• Several dipole and quadrupole prototypes are built and
mapped, still trying to reach our specifications.
Wien Filter Design• A Wien filter deflects all
particles except those with a velocity of E/B.
• Homogeneous fields are extremely important, and it is critical to maintain the E/B ratio in the fringe field region.
• The field clamp and the special design of the electrodes insure control of the fringe field.
Wien filter fields – transverse
Need good E and B field homogeneity in the center
|∆B/B|<0.2%
Magnetic field in the filter center
6.6 cm
5 cm
Y
X
Z=WFlength/2
100kV
∆E/E|<0.2%
6.6 cm
5 cm
Y
X
Wien filter magnetic fringe field
Without field clamp With field clamp
Z
ZZ
YY
Mobile field clamp tobe fine adjusted using thebeam
Wien filter electrostatic fringe field
Clamped magnetic field
E-field of standard WF
electrodes
E-field of optimized WF
Wien filter fringe fields - longitudinal
-5.0000E-01
0.0000E+00
5.0000E-01
1.0000E+00
1.5000E+00
-600 -500 -400 -300 -200 -100 0 100 200 300 400
Magnetic fieldElectric fieldE/B ratioFigure of merit
Modified Wien filter fringe field
Aberrations correction
Optic calculation up to 4th orderCorrections up to 3rd orderembedded in the magneticdipoles pole faces.
The goal is to minimize spotsize at the mass selectionslits
Horizontal projection
Detection• Now that the charge and mass selection
is done:– Detection … – Yes but: Background from beam scattered and
charge exchanged
Charge selection
Mass selection
Additional cleaning
Detection system
Particle ID to have additional selection
Tof vs Energy (Micro-channel plate + Si)Ionization chamber
Localize eventual background trajectories
• In other recoil separators dedicated to radiative capture, background is ~10-12 relative to initial beam intensity– Difficult to simulate:
• Enough particles• All the effects (scattering and charge exchange) at every
position
• Procedure to identify what part of the beam phasespace reaches the focal plane detector– Backtracking of beam particle filling the face space
Possible background trajectories
18O(α,γ)22Ne4+ @ 2.MeV
18O4+ @ 2.±0.08MeV
Impact on the NSL
• Significant infrastructure improvement– Electric power
• increase available power in the lab– Water cooling
• de-ionized chilled water– Air conditioning
• Temperature stability in the target room– Concrete block moving
Reduced availability of the KN/JN target room
RMS Infrastructure
RMS Infrastructure
RMS Infrastructure
Dry Coolers on Roof
Timeline?
• Who knows?• Bruker is far behind schedule, but we hope
to have magnets by early spring 2009.• Installation and commissioning during the
summer and fall.• New accelerator? New tower for new
accelerator? When? Before RMS is finished?