Reaction rates in the Laboratory

Transcription

1 Reaction rates in the Laboratory Example I: 14 N(p,γ) 15 O slowest reaction in the CNO cycle Controls duration of hydrogen burning Determines main sequence turnoff glob. cluster ages stable target can be measured directly: γ-ray detectors Accelerator vacuum beam line Proton beam N-target Faraday cup to collect charge but cross sections are extremely low: Measure as low an energy as possible then extrapolate to Gamow window

2 Calculating experimental event rates and yields beam of particles hits target at rest area A j,v thickness d assume thin target (unattenuated beam intensity throughout target) Reaction rate (per target nucleus): Total reaction rate (reactions per second) λ = σ j R =λ Adn T = σ Idn T with n T : number density of target nuclei I=jA: beam number current (number of particles per second hitting the target) note: dn T is number of target nuclei per cm 2. Often the target thickness is specified in these terms.

3 Events detected in experiment per second R det R = Rε det ε is the detection efficiency and can accounts for: detector efficiency (fraction of particles hitting a detector that produce a signal that is registered) solid angle limitations absorption losses in materials energy losses that cause particles energies to slide below a detection threshold

4 14 N(p,γ) level scheme Gamow window 0.1 GK: kev γ 0 Direct gs capture ~7297 kev + E p γ-signature of resonance 6791 kev

5 LUNA Laboratory Underground for Nuclear Astrophysics (Transparencies: F. Strieder Gran Sasso Mountain scheme 1:1 Mio cosmic ray suppression

6 Spectra: above and under ground

7 Beschleuniger bild

8 Setup picture

9 Spectrum overall

10 Spectrum blowup

11 Results: Gamow Window Formicola et al. PLB 591 (2004) 61 New S(0)= kevb (NACRE: )

12 New Resonance? Resonance claim and TUNL disproof

13 Effect that speculative resonance would have had

14 Example II: 21 Na(p,γ) 22 Mg problem: 21 Na is unstable (half-life 22.5 s) solution: radioactive beam experiment in inverse kinematics: 21Na + p 22Mg + γ thick 21Na production target hydrogen target 22Mg products Accelerator I Accelerator 2 p beam ion source 21 Na beam γ-detectors difficulty: beam intensity typically /s (compare with 100 μa protons = 6x10 14 /s) particle identification so far only succeeded in 2 cases: 13N(p,γ) at Louvain la Neuve and 21Na(p,γ) in TRIUMF (for capture reaction)

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17 TRIUMF

18 Results Result for 206 kev resonance: S. Bishop et al. Phys. Rev. Lett. 90 (2003) 2501

19 Example III: 32 Cl(p,γ) 33 Ar Shell model calculations Herndl et al. Phys. Rev. C 52(1995)1078 proton width strongly energy dependent rate strongly resonance energy dependent

20 NSCL Coupled Cyclotron Facility H. Schatz

21 Installation of D4 steel, Jul/2000

22 Fast radioactive beams at the NSCL: low beam intensities Impure, mixed beams high energies ( MeV per nucleon) (astrophysical rates at 1-2 MeV per nucleon) great for indirect techniques Coulomb breakup Transfer reactions Decay studies

23 H. Schatz Setup S800 Spectrometer at NSCL: Focal plane: identify 33Ar 34Ar Plastic d 33Ar excited 33 Ar Beam blocker 34 Ar 34 Ar Radioactive 34 Ar beam 84 MeV/u T 1/2 =844 ms (from 150 MeV/u 36 Ar) Plastic target SEGA Ge array (18 Detectors) People: D. Bazin R. Clement A. Cole A. Gade T. Glasmacher B. Lynch W. Mueller H. Schatz B. Sherrill M. VanGoethem M. Wallace

24 SEGA Ge-array S800 Spectrometer

25 H. Schatz New 32 Cl(p,γ) 33 Ar rate Clement et al. PRL 92 (2004) 2502 Doppler corrected γ-rays in coincidence with 33Ar in S800 focal plane: γ-rays from predicted 3.97 MeV state stellar reaction rate Ar Ar level level energies energies measured: measured: 3819(4) 3819(4) kev kev (150 (150 kev kev below below SM) SM) 3456(6) 3456(6) kev kev (104 (104 kev kev below below SM) SM) reaction rate (cm 3 /s/mole) with shell experimental model only data x 3 uncertainty x10000 uncertainty temperature (GK) Typical X-ray burst temperatures

26 NSCL ReA3 Fast beams Gas cell

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28 Science with CCF reaccelerated beams Rates in pps > direct (p,γ) direct (p,α) or (α,p) transfer (p,p), some transfer and p-process Up to here: For indirect measurements: many For direct measurements: some important rates Capabilities: sufficient beam intensities for many important measurements all beams available once system commissioned probably very good beam purity none of the measurements identified can be performed at another facility as of now

29 Overview of the FRIB Layout

30 ReA12 and Experimental Areas A full suite of experimental equipment will be available for fast, stopped and reaccelerated beams New equipment Stopped beam area (LASERS) ISLA Recoil Separator Solenoid spectrometer Active Target TPC

31 Science with reaccelerated beams at FRIB Rates in pps 10 > most reaction rates up to ~Sr can be directly measured Direct measurements for many (α,γ) reactions in p-process All reaction rates can be indirectly measured including 72 Kr waiting point All reaction rates up to ~Ti can be directly measured Very strong nuclear astrophysics science case