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