S1378: Lifetime Measurement of the MeV State in 23 Mg

Transcription

1 S1378: Lifetime Measurement of the MeV State in 23 Mg Charles Akers, Peter Bender, Alison Cheeseman, Greg Christian, Randy Churchman, David Cross, Barry Davids, Lee Evitts, Jennifer Fallis, Naomi Galinski, Adam Garnsworthy, Greg Hackman, Steffen Ketelhut, Jonathan Lighthall, Oliver Kirsebom, Peter Machule, David Miller, Chantal Nobs, Chris Pearson, Allison Radich, Mustafa Rajabali, Alex Rojas, Alisher Sanetullaev, Carl Unsworth, Chris Ruiz, and Chris Wrede TRIUMF 21 Nov 2012

2 Classical novae periodicities range typically between yr). Both novae and supernovae are characterized by a remarkable energy output, with peak luminosities reaching 10 4 and L, respectively. A basic difference between both explosive phenomena is the amount of mass ejected (the whole star in a thermonuclear supernova versus M in a nova) as well as the mean ejection velocity (> 10 4 km s 1 in a supernova, and several 10 3 km s 1 in a classical nova). Z Model 1.35 M (50% ONe enrichment) T = K T ρ = g cm peak -3 ε nuc = erg g -1 s -1 ΔM env = M Si Al Mg Na C B Be Li He H O N Ne F N P S Ar Cl K Ca (p,γ) (p,α) (β + ) Log T = (Reaction 1 x Fluxes) 7 K : -2 : -3 : -4 : -5 : -6 : -7 Fig. 1. The main nuclear activity, shown in terms of reaction fluxes (number of reactions per unit volume J. José (2010) and time), at Tpeak, for a nova model of 1.35 M, with 50% ONe enrichment. The dominant nuclear reaction flow proceeds close to the valley of stability and is dominated by (p,γ), (p,α), and β-decays. See text for details.

3 1275 kev γ-ray line Na t 1/2 = 2.6 yr β kev γ Ne g.s.

4 Main destruction mechanism: 22 Na(p,γ) 23 Mg Two direct measurements both show that the reaction is dominated by a resonance at 206 kev (E x = 7786 kev, J π = 7/2 + ). But, the resonance strengths differ by a factor of 3: ωγ = 1.8 ± 0.7 mev Bochum [1] ωγ = mev Seattle [2] [1] Stegmüller et al., NPA 601, 168 (1996) [2] Sallaska et al., PRL 105, (2010); PRC 83, (2011)

5 Indirect determination of ωγ p 7/ kev kev 22 Na γ(7335 kev) γ(5734 kev) 7/2 + 5/2 + 3/ Mg 2052 kev 451 kev g.s. ωγ = 2J + 1 (2J a + 1)(2J A + 1) Γ p Γ γ Γ p + Γ γ

6 Indirect determination of ωγ Total width: Γ = Γ p + Γ γ Proton branching ratio: B p = Γ p /Γ Γ p Γ γ Γ p + Γ γ = B p (1 B p )Γ B p determined in recent 23 Al β-decay experiment [3], B p = ± Γ deduced from the lifetime, τ, determined in a recent 12 C( 12 C,n) 23 Mg experiment [4], using the doppler-shift attenuation method, τ = 10 ± 3 fs [3] Saastamoinen et al., PRC 83, (2011) [4] Jenkins et al., PRL 92, (2004)

7 Experimental situation ωγ = mev direct (Bochum) ωγ = mev direct (Seattle) ωγ = mev indirect

8 Doppler-shift attenuation method (DSAM) Principle: γ Previous experiment at TRIUMF ( 19 Ne) LIFETIMES OF STATES IN 19 Ne ABOVE THE... Mythili et al., PRC 77, (2008) 60 (a) γ Counts / 2.5 kev γ Energy (kev) τ = 1 fs 10 ps nts / 2.5 kev (b) τ = 7.1 ± 1.9 ± 0.6 fs

9 Experimental setup MeV Au 25 µm collimator 15 o ΔE 100 µm GRIFFIN Ge detector 3 He E 500 µm 3 He + 24 Mg α + 23 Mg E rel = 8.3 MeV, Q = 4.0 MeV 25 µm Au foil 3 He implantation depth = 0.1 µm, area density = cm 2 Clean γ-ray spectrum by gating on α particles.

10 Experimental setup

11 Run overview Net beam time = 4.8 days (74%) Av. FC0 = 1.6e10 pps (request 3.0e10 pps, min 0.6e10 pps) Av. R = 68.5 Ω ( 79.5 C) Av. P = 1.2e-7 torr scheduled beam time FC0, Event rate, P R FC0 [na] Event rate [per 0.1s] R [ohm] P [1e-8 torr] Nov 04 Nov 05 Nov 06 Nov 07 Nov 08 Nov 09 Nov 10 Nov 11 Nov 12 Nov 13 Nov 14 Nov

12 E-E plot ΔE (MeV) E (MeV)

13 Gate on 3 He loci Events / 2 kev t 1/2 =1.3 ps 24 Mg 1369 kev g.s. E γ (kev)

14 α-particle energy spectrum α 2 α 3 α 1 E+ΔE (MeV)

15 Known states in 23 Mg below 8 MeV

16 Known states in 23 Mg below 8 MeV α 3 α 2 α 1

17 Gate on all α particles Events / 2 kev 197 Au(279 gs) 23 Mg(451 gs) Au(548 gs) E γ (kev)

18 Gate on α 2 Events / 2 kev ₓ transition in 23 Mg ₓ not identified (yet) ₓ ₓ ₓ E γ (kev)

19 Gate on α 3 Events / 2 kev transition in 23 Mg E γ (kev)

20 Identified γ transitions between low-lying states in 23 Mg

21 23 Mg( ) transition E γ E γ (1 + v/c) 7335 kev ( ) = 7815 kev Solid angle δe γ ±7 kev Attenuation δe γ 9 kev (assuming τ = 10 fs) region of interest in gamma spectrum ( kev) background region ( kev) gamma counts δe gate (MeV)

22 23 Mg( ) transition signal 30 backgr 10 Events / 4 kev E γ (kev)

23 Summary The 1275 kev γ-ray line from 22 Na is considered the best known test of novae models Main destruction mechanism: 22 Na(p, γ) The two direct determinations of the 22 Na(p, γ) rate disagree A new measurement of the lifetime of the 7786 kev level is required to establish confidence in the indirect determination of the rate S1378: Attempt to determine lifetime using the DSAM method 75 MeV 24 Mg beam on 3 He implanted Au foil; coincident detection of α particles and γ rays at 0 Preliminary result: γ line observed, but statistics are not overwhelming

24 Doppler shift E γ = E γ (1 + β cos θ) In the limit t t slow down, β(t) Insert t = τ = 10 fs, M = 23 u, and (de/dx) t=0 = 8.0 MeV/µm a, t ( ) de Mc dx t=0 β(τ) Estimated variation in doppler shift: E γ E γ = MeV = 8.6 kev a stopping power of 45 MeV 23 Mg ions in Au