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The 26gAl(p,)27Si Reaction at
DRAGON
Heather CrawfordSimon Fraser University
TRIUMF Student SymposiumJuly 27, 2005
Outline• The astrophysical importance of the
26gAl(p,)27Si reaction• Overview of the DRAGON facility and its
role in astrophysics at TRIUMF • The principle of direct measurement of
the resonance strength of this reaction using inverse kinematics at DRAGON
• Methods of beam normalization and its importance in calculating resonance strength
Why Study Astrophysics?
• All elements are produced through nuclear reactions
in stars and novae or supernovae explosions;
most are radiative capture reactions (i.e.
(p,), (n,) or (,))• Astrophysics studies these
reactions, to understand the origin of the chemical elements… to understand
where we come from!
Astrophysical Importance of 26gAl(p,γ)27Si Reaction• 26gAl is directly observable in supernovae by orbiting
gamma telescopes, due to the characteristic gamma emitted during its beta decay
• This allows comparison of observed nuclear abundances with values calculated using theoretical models
• Accurate models require good knowledge of resonance energies and strengths
Mg-Al System
26Si
26Mg24Mg 25Mg
27Al26Al25Al
28Si27Si
Characteristic1.809 MeV
Gamma Ray from 26gAl Decay
4.16s
Only direct
method of destruction for 26gAl aside from
its beta decay is radiative proton capture
7.18s 0.717 Myr
Detector of Recoils And Gammas Of Nuclear Reactions (DRAGON)• DRAGON is a recoil mass separator used in the study of (p,γ) and (α,γ) reactions
DRAGON Gas Target and BGO Gamma Array
• Windowless gas target maintains H2 or He gas at 4-8 Torr
• Surface barrier detectors within target detect elastically scattered protons
• 30 BGO detectors surround target and detect prompt gammas during reactions
DRAGON Separator• DRAGON uses
two stage separation; improves beam suppression and reduces background
• 26gAl(p,)27Si: separate 4+ 27Si recoils from the beam and contaminants
First Stage
Second Stage
DRAGON End Detectors• 26gAl(p,γ)27Si:
MCP (micro-channel plate) used in conjunction with a DSSSD (double-sided
silicon strip detector) • DSSSD gives number, energy, position and local timing information (with the MCP)
• MCP produces a signal as ions pass through (timing signal)
• Other experiments at DRAGON make use of an ion chamber, which gives particle ID information, timing and energy information
Direct Measurement of 26Al(p,)27Si Reaction Using Inverse Kinematics• Intense radioactive 26Al beam is incident on 6 Torr
H2 target with approximately 202 keV/u of energy
• Particles pass through target reaching resonance energy (188 keV in center of mass frame) near middle of target -- some react to produce 27Si recoils, most pass straight through
• Recoils emerge with ~ same momentum as beam, with a small angular spread ( ~ 15 mrad)
26Al Gas Target
H2 27Si
γ
Reaction Rate and Resonance Strength• Cosmic reaction rates are dominated by narrow
resonances which occur within the Gamow window
M
mM1
2 Yield
2
• Narrow resonances are characterized by a resonance strength, ωγ
• Resonance strength is determined by measuring the thick target yield, given by the following equation:
M = mass of target, m = mass of projectile, = de Broglie wavelength, = stopping power
Requirements for Thick Target Yield Determination
Determination of the thick target yield requires accurate knowledge of:
1) Number of reactions that occur, determined through γ-heavy ion
coincidence → 11 recoils were observed over the
entire 3 week run 2) Number of beam particles incident on the
gas target, which requires…Beam Normalization
Beam Monitors within DRAGONWithin DRAGON are a number of potential
beam ‘monitors’:•Rutherford scattered protons in gas target
•Current on the left mass slit
•Faraday cup readings
•Beta monitor
•Contamination NaI and HPGe monitors at the mass slit box
•Leaky beam on the DSSSD
Evaluating Possible Beam Monitors for Normalization
Current on Mass Slit Left as a Function of Time for Run 15018
-100
0
100
200
300
400
500
5:02:24 5:16:48 5:31:12 5:45:36 6:00:00 6:14:24 6:28:48 6:43:12 6:57:36 7:12:00
Time
Mas
s S
lit L
eft C
urr
ent (
epA
)
• Beta monitor, contamination detectors and DSSSD: of little use for beam normalization
• Current of Left Mass Slit: good beam variation profile, but prefer alternative• Scattered
Proton Monitor: excellent
monitor when properly set
• Faraday cup upstream of target: best
measurement of absolute
beam intensity
Beam Normalization to Faraday Cup Reading
•Faraday cup readings are most reliable -- normalize other monitors
to faraday cups
•Use values near beginning and end of each run to establish a
normalization factor
•Integrate monitor responses over entire run, and use the
normalization factor to determine the equivalent integrated response
on the faraday cup
• # scattered protons # of incident beam particles,
gas pressure, 1/T2
Rutherford Scattering into Surface Barrier Detectors in Gas TargetRutherford’s Formula:
• Normalization factor can be determined that is independent of beam energy and gas
pressure, defined as below:
2/sin
1
4
1
4 AreaParticles/ Gas # Particles Beam # Protons Scattered #
42
2
0
2
aT
zZe
2aTProtons Scattered
Pressure
6
Current FC R
e
t
Rutherford Scattering into Surface Barrier Detectors in Gas Target
• Take Δt to be 300 seconds, and look at SB trigger rate; if ~ constant, calculate R with
integral of proton peak for first 300 seconds, and FC reading from beginning of run.R = 1.320 × 103 26Al·Torr/{proton·(keV/u)2}
Rutherford Scattering into Surface Barrier Detectors in Gas Target
• Use this value of R with integral over entire run to determine number of incident beam
particles.
Integral = 158384
# 26Al particles over entire run = 1.42 × 1012
Left Mass Slit Beam Monitor• Left mass slit reads a current due to a portion of the
beam being deposited there• MIDAS records the current reading every 30 seconds• Establish normalization factor using ratio of FC to
left mass slit readings at beginning and end of each run
Left Mass Slit Current as a Function of Time
-50.00
0.00
50.00
100.00
150.00
200.00
250.00
300.00
5:16:48 5:45:36 6:14:24 6:43:12 7:12:00 7:40:48 8:09:36 8:38:24
Time
Lef
t M
ass
Sli
t C
urr
ent
(ep
A)
Start RunFC4 = 120 epA
End RunFC4 = 145 epA
Mass Slit Left = 198.73 epA Mass Slit Left = 242.68 epA
Average Normalization Factor = 0.601
Left Mass Slit Beam Monitor
Left Mass Slit Integration
0.00E+00
5.00E-11
1.00E-10
1.50E-10
2.00E-10
2.50E-10
3.00E-10
5:29
:41
5:35
:11
5:40
:41
5:46
:11
5:51
:41
5:57
:11
6:02
:41
6:08
:11
6:13
:41
6:19
:11
6:24
:41
6:30
:11
6:35
:41
6:41
:11
6:46
:41
6:52
:11
6:57
:41
7:03
:11
7:08
:41
7:14
:11
7:19
:41
7:25
:11
7:30
:41
7:36
:11
7:41
:41
7:47
:11
7:52
:41
7:58
:11
8:03
:41
8:09
:11
8:14
:41
8:20
:11
8:25
:41
Time
Lef
t M
ass
Sli
t C
urr
ent
(ep
A)
Integrated Area = 2.29E-06 Coulombs
• Integrate left mass slit values over entire run …
• Then multiply by normalization factor to find integrated charge on faraday cup in coulombs, and convert to 26Al particles…
# 26Al particles on target = 1.43 × 1012
Comparing Normalization Methods…
• Comparing the two methods, we see a difference in this case of less than 1%
• For over 60 runs where both methods were used, the average difference was ~ 5%
→ When one method cannot be applied, the other method can be trusted to yield an accurate normalized beam
Rutherford Scattered
Proton MonitorLeft Mass Slit
# 26Al particles incident on target (15094):
1.43 × 1012 1.42 × 1012
What’s Next?
• Calculation of the resonance strength,
ωγ
• DRAGON has requested additional
beam time for 26gAl(p,γ)27Si to
reduce the error on the experimental
resonance strength
Summary• A good knowledge of the 26gAl(p,γ)27Si
reaction is important in developing models for the production of 26gAl
• Cosmic reaction rates are determined by narrow resonance reactions; these are
characterized by a resonance strength, ωγ
• Resonance strength can be determined directly by measuring thick target yields
using DRAGON
• Beam normalization is critical to determining thick target yield
• There are a number of ways to normalize beam, which provide results in very good
agreement with one another
Thanks to the DRAGON group