$β$-Decay Half-Life of the $rp$-Process Waiting Point Nuclide $^{84}$Mo

0.5 MB·English

by  J. B. Stoker

β-Decay Half-Life of the rp-Process Waiting Point Nuclide 84Mo
J. B. Stoker1,2, P. F. Mantica1,2, D. Bazin2, A. Becerril2,3,4, J. S. Berryman1,2,
H. L. Crawford1,2, A. Estrade2,3,4, C. J. Guess2,3,4, G. W. Hitt2,3,4, G. Lorusso2,3,4,
M. Matos2,4, K. Minamisono2, F. Montes2,4, J. Pereira2, G. Perdikakis2,
H. Schatz2,3,4, K. Smith2,3,4, R. G. T. Zegers2,3,4
(1) Department of Chemistry, Michigan State University, East Lansing, Michigan 48824
(2) National Superconducting Cyclotron Laboratory,
Michigan State University, East Lansing, Michigan 48824
(3) Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824 and
(4) Joint Institute for Nuclear Astrophysics, Michigan State University, East Lansing, Michigan 48824
(Dated: January 8, 2009)
A half-life of 2.2 ± 0.2 s has been deduced for the ground-state β decay of 84Mo, more than 1σ
9 shorterthanthepreviouslyadoptedvalue. 84Moisaneven-evenN =Z nucleuslyingontheproton
0
dripline, created during explosive hydrogen burning in Type I X-ray bursts in the rapid proton
0
capture (rp) process. The effect of the measured half-life on rp-process reaction flow is explored.
2 Implications on theoretical treatments of nuclear deformation in 84Mo are also discussed.

n
a
J I. INTRODUCTION the modeling of X-ray bursts. Nevertheless, the amount

8 of ejected materialis still being debated and the compo-
sition of material produced in an rp-process event relies
A. The Astrophysical rp-Process
] on experimental data, and in instances where these data
x
e are lacking,nuclear structure models [6]. The half-life of
- The present model for the rp-process begins with the 84Moisanecessaryexperimentalparameterformodeling
cl accretion of hydrogen and helium rich matter onto neu- reactionflowabovemass84,anddirectlydetermines the
u tron stars from nearby companion stars. Gravitational amount of 84Sr formed during X-ray bursts.
n energyisreleasedintheformofX-raysasmatterreaches
[ the intimate regions of the neutron star. The matter

1 is compressed as it forms an accretion disk and trav-
B. Shape Deformation Along N =Z
v els through the gravitational field gradient towards the
8 neutron star, which eventually results in thermonuclear
4 burning. The rp-process is a sequence of (p,γ) reactions Deformed nuclei in general exhibit different single-
0 and subsequent β+ decays up to 107,108Te [1]. The pro- particle level spacing and higher level densities than
1 cess begins at 41Sc, with the seed nuclei produced by spherical nuclei. Single-particle levels, level densities
.
1 a series of fast (α,p) and (p,γ) reactions on CNO-cycle and nuclear masses are important ingredients for cal-
0 nuclei. The composition of the accreted matter governs culating proton-capture rates and β-decay half-lives [7].
9 theoveralldurationandpeaktemperatureofthenuclear The ratio of the 4+ and 2+ yrast state energies [R4/2 ≡
0 burning stage of X-ray bursts [2]. E(4+)/E(2+)] can be used as an indicator of shape de-
: 1 1
v The rp-process circumstances prove plausible for the formation in even-even nuclei, with a smaller value rep-
i resenting a less deformed nucleus [8]. The R trend
X synthesisofstable isotopesontheproton-richside ofthe 4/2
nuclear chart (p-nuclei) below A = 107. This is because for even-even nuclei along N = Z reveals a deformation
ar the rp-process ashes are finally determined by β decay maximumat76Srand80Zr(seeFig.1)[9,10,11]. AR4/2
fromthe proton-richsideofβ-stability[2]. Theobserved ratioof2.52for84Momarksthebeginningofatransition
abundances forthe p-nuclei92,94Mo and96,98Ru arecur- towards the believed spherical, doubly magic 100Sn.
rently at odds with abundance predictions using other Theoretical predictions for the β-decay half-life of
solarproductionmechanisms[3]. Therp-processcanpro- 84Mo within the Quasi-particle Random Phase Approxi-
ducethesenucleiundercertaincircumstances,sustaining mation(QRPA) vary from 2.0 s by Sarrigurenet al. [12]
X-rayburstsasapossiblecontributortotheirsolarabun- to 6.0 s by Biehle et al. [13]. Fig. 2 shows a comparison
dances. The amount of rp-process material emitted into of the half-lives calculated by Sarriguren et al. (QRPA-
the interstellarmediumdepends onthe isotopicoverpro- Sk3, QRPA-SG2) and Biehle et al. (QRPA-Biehle) with
duction factor, the frequency of burst occurrences that experiment. The principle difference between these the-
produce p-nuclei, and the fraction of material that es- oreticaltreatises is the set of nuclei usedto calibrate the
capes the gravitational field of the neutron star [4]. It self-consistentinteractionparametersforparticle-particle
has been pointed out that the high gravitational field in coupling strength and nuclear deformation.
thevicinity ofneutronstarshindersstellarejectionofall The QRPA-Biehle prediction relies on the nearby nu-
butthemostenergeticash[5]. Thebulkofnucleisynthe- clei 88,90Mo, 92Ru, and 94Pd, which do not exhibit the
sizedintherp-processwouldnotescapethegravitational samelevel(2.1≤R ≤2.23)[14,15,16,17]ofdeforma-
4/2
field. This limited mass ejection is a majordrawbackfor tion observed in the N = Z region near A = 80 but ap-
2
100
10
)
s
(
2
1/
T
1 QRPA-Sk3
QRPA-SG2
QRPA-Biehle
Experiment
ThisWork
FIG.1: Yraststatesupto8+ineven-evenN =Znuclei. The 0.1
transition energies are given in MeV. R ratios are shown
4/2 64Ge68Se72Kr 76Sr80Zr84Mo 88Ru 92Pd
just below 0+ state. Taken from [10, 11].
FIG. 2: (color online) Half-lives of even-even N = Z nuclei
forA=64toA=92(filled circles) comparedtocalculations
proach spherical shapes, lengthening the predicted half- usingtheQRPA.Detailsofdifferenttheoreticalself-consistent
lives. The self-consistent parameters for the QRPA-Sk3 parametersaregiveninthetext[12,13]. Thenewlymeasured
result for 84Mo is shown as theopen circle.
and QRPA-SG2 cases were derived from experimental
data fromnuclei in the regionof interest, and mostly re-
produce the experimental half-lives using self-consistent
II. EXPERIMENTAL PROCEDURE
deformations that minimize the energy. The previously
measured half-life of 84Mo reported by Kienle et al. [18]
falls between the two calculations. This would imply a Nuclei for this study were producedvia fragmentation
level of deformation unique to the mass region, perhaps of 124Xe projectiles accelerated to 140 MeV/nucleon in
inconsistent with the observed trend of measured R the coupled K500 and K1200 cyclotrons at National Su-
4/2
ratios along even-even N =Z nuclei. perconducting Cyclotron Laboratory. The 124Xe beam
Recent re-measurements of the half-lives of 80Zr [19] was impinged upon a 305 mg/cm2 9Be target. Frag-
and 91,92,93Rh [20] are shown in Table I. Each of these ments were transported through the A1900 Fragment
Separator for separation[21]. The A1900 dipole settings
measurements are background suppressed by γ-ray gat-
ing and are systematically lower than those reported by wereBρ1,2 =2.9493Tm,andBρ3,4 =2.5635Tmwitha
180mg/cm2 Al wedge placed at the intermediate image.
Kienle et al. [18], at times by more than 1σ. A similarly
shorter resultfor a 84Mo re-measurementwould validate Fragmentswithin±0.5%ofthe centralmomentumwere
transported to the A1900 focal plane.
Sarriguren’s approach for N = Z nuclei in this region,
The momentum distributions of nuclei produced in
and give consistency with the deformation implied by
intermediate-energy fragmentation reactions are asym-
the measured R .
4/2
metric. In Fig. 3 is shown a simulation of yields as a
functionofBρfortheprincipleisotopesproducedinthis
study. The simulation was performed with the program
TABLEI:Experimentalhalf-livesdeterminedbyKienleetal. LISE[22]. Theproductionratesfromthelow-momentum
[18] compared with β-γ correlated measurements [19, 20]. tails of more stable species exceeds that of the exotic
Nucleus Jπ T (s) a β-γcoincidence T (s) 84Mo such that a decay experiment would not be fea-
1/2 1/2
sible without additional beam purification beyond that
80Zr (0+) 5.3+1.1 4.1+0.8 b achieved with the A1900.
−0.9 −0.6
93Rh (9/2+) 13.9+1.6 11.9±0.7 c ThenewRadioFrequencyFragmentSeparator(RFFS)
−0.9
92Rh (≥6+) 5.6+0.5 4.66±0.25 c was implemented at NSCL to purify beams of neutron-
−0.5
91Rh (9/2+) 1.7+0.2 1.47±0.22 c deficient nuclei [23]. The device consists of two horizon-
−0.2
tally parallel plates 1.5 m long, 10 cm wide, and 5 cm
aRef.[18]. apart. A time-varying electric field is applied across the
bRef.[19].
cRef.[20]. plates with an amplitude up to 100 kV peak-to-peak. A
voltageof47kVpeak-to-peakwasappliedforthisstudy.
This field varied sinusoidally at a frequency of 23.145
3
FIG. 4: (color online) Plot of thevertical beam position as a
FIG.3: (coloronline)Simulationoftheyieldsinparticlesper
functionofTOF.Therectanglehighlightstheregionaccepted
second (pps) as a function of Bρ for the principle isotopes
through the10 mm slit setting.
created in thisstudy.
MHz, matched to the K1200 cyclotron. The time re- eragetimebetweenimplantationsintoasinglepixelthat
quired for nuclei in the study to pass through the full exceeded the correlation time, which is set in software
1.5-m length of the RFFS was roughly equal to 1/3 of a based on the expected half-life of the isotope of interest.
RFperiod. TheTime-Of-Flight(TOF)separationofthe The distribution of 84Mo fragments on the surface of
beamspecies resultedineachfragmentseeing adifferent the DSSD is presented in Fig. 5. As noted above, back-
1/3 of the sinusoidal voltage cycle, and therefore receiv- ground is controlled in the experiment by taking advan-
ingeachadifferentvertical“kick”. Adiagnosticboxwas tage of the high degree of segmentation of the DSSD.
located6mdownstreamofthe RFFSexitandcontained Here the 84Mo fragments are distributed over ∼ 1/4 of
an adjustable slit system and particle-counting detec- the DSSD surface, and overlapin position most strongly
tors. Two retractable parallel-plane avalanche counters with 80Y ions but not 79Sr ions, where 80Y and 79Sr
(PPACs)wereplacedalongthebeampath,oneupstream are two of the three significant contaminants in the sec-
of the slit and the other downstream. The vertical de- ondarybeam. Theothersizeablecontaminant,73As,will
flection of the fragments at the slit position is presented haveminimalimpactontheaccuratecorrelationof84Mo
in Fig. 4. The deflection range selected by the 10 mm β-decayevents,sinceitshalf-lifeisoforderdays(seeTa-
slit gap is also indicated. The phase of the RFFS can be ble II). The percentage of 84Mo implantations that were
delayed relative to the cyclotron RF such that the frag- preceeded by a 80Y implantation in the same pixel was
ment of interest appears at a trough, peak, or node of evaluated for the 10-s correlation time used to establish
the sinusoid as desired. the decay curve for regression analysis as outlined be-
Table II quantifies the overall and isotope specific re- low. These so-called “back-to-back” implantations oc-
jection factors observed during this study. The table curred for ∼ 8% of the 84Mo implantation events, and
demonstrates the importance of distinguishing the de- were essentially distributed equally in time as the av-
vice’s selective rejection rate as opposed to the overall erage time between implantations in a given pixel well
rejection rate. Selective rejection of key contaminants exceeded the correlationtime. The impact of the “back-
78Rb and 77Kr was of order 104, while the counting rate to-back”84Mo-80Yimplantationsis furthermitigatedby
of the fragment of interest 84Mo was not affected. The the fact that 80Y has two β-decaying states as listed in
overall rejection factor was 180. Fragments that passed TableII. Theisomeric1−stateof80Ydecayswithahalf-
through the RFFS slit system were delivered to the ex- life of 4.8±0.3 s, and is a more significant contributor
perimental end station for analysis. tothebackgroundanalysisofthe84Moβ decaythanthe
The Beta Counting System (BCS) [24] was used longer-lived30.1±0.5 s ground state with Jπ =4− [25].
for event-by-event correlation of fragment implantations β decay of the 1− isomeric state contributed only ∼10%
with their subsequent β-decays. The system employed tothetotal80Yactivity,determinedfromanalysisofthe
a Double-Sided Silicon Strip Detector (DSSD), a single delayed γ-ray spectrum of 80Y collected in the present
995 µm × 4 cm × 4 cm silicon wafer segmented into 40 work.
1-mm strips in both the x and y dimension, providing Prompt and delayed γ rays were monitored at the ex-
1600 individual pixels. The high degree of segmentation perimental end station with 16 Ge detectors from the
and beam filtering were both necessary to realize an av- Segmented Germanium Array (SeGA) [26]. The detec-
4
TABLE II: Isotope specific and overall beam rejection
40
rates. The rejection factor is reported as the ratio of
RFFS /RFFS for individual fragment yields. Rejection
off on
factorslargerthan1indicateareducedtransmissionwiththe 30
RFFS on. Rates for specific isotopes are given in particles
y
per second per particle nanoamperes of beam, normalized to
the measured rate of 84Mo. The beam intensity was attenu- 20
ated when RFFS was off (V = 0 kV) to preserve Si detector
longevity. 79
10 Sr
a
Rate
V = 0 kV V = 47 kV Rejection Half-life
50 mm slit 10 mm slit Factor 0
0 10 20 30 40
84Mo 1 1 1 3.7+1.0 s x
−0.8
83Nb 15 16 1 4.1±0.3 s
82Zr 80 40 2 32±5 s 40
81Zr 20 10 2 5.3±0.5 s
80Y 130 200 0.6 30.1±0.5 s
30
4.8±0.3 s
79Sr 4000 85 47 2.25±0.10 min y 84
78Rb 18700 0.4 46700 17.66±0.08 min 20 Mo
5.74±0.05 min
77Kr 13500 0.3 45000 77.4±0.6 min
76Br 1150 15 77 16.2±0.2 h 10
1.31±0.02 s
74Se 1980 5 400 stable
0
73As 700 630 1.1 80.30±0.06 d
0 10 20 30 40
b
Sum 83 0.5 180
x
I 0.8 pnA 10 pnA
beam
aRatesnormalizedto84Mo,5×10−4 pps/pnA 40
bAbsoluterateinpps/pnA
30
torsweremountedonaframedesignedtocloselypackthe 80
y
cylindricalGe crystalsintworingsof8detectorsaround Y
20
theBCSchamberforγ-raydetection. Eachdetectorwas
mounted with its cylindrical axis parallel to the beam
axis. The DSSD was located in the plane that separated 10
theupstreamanddownstreamGedetectorrings,thereby
maximizingtheoveralldetectionefficiencyofγ-raysemit-
0
ted from nuclei implanted in the DSSD.
0 10 20 30 40
Ions implanted in the DSSD were monitored by loca-
x
tion for positron emission in the implantation pixel, as
well as the four nearest-neighbor pixels, over a correla-
tion time of 10 s. A correlationefficiency for β detection
of ≈ 38% was realized.
Three Si PIN detectors (PIN1-3) placed upstream of FIG. 5: (color online) Distribution of 79Sr, 84Mo, and 80Y
fragments in the DSSD. The vertical deflection of fragments
the DSSD servedas active degraders. PIN1 wasused for
bytheRFFS based on time-of-flight producesa non-uniform
the ∆E and TOF start signal during particle identifica-
distribution of fragments in the y dimension.
tion(PID).Theactivedegraderthicknesseswereselected
such that fragments were stopped in the front 1/3of the
DSSD, increasing the probability of detecting the small
∆E of a β-particle emitted in the downstream direction. in x and y.
DownstreamoftheDSSDweresix5cm×5cmSingle- The DSSD provided a trigger for the master gate on
SidedSiliconStripDetectors (SSSD1-6),whichwerepri- either an implantation or decay event that registered a
marily used to veto signals from light particle that were coincidence signal in the front and back strips. SeGA
transported to the end station. Each SSSD was seg- readoutwasopenedfor20µsfollowingeacheventtrigger,
mentedintosixteenstripsononefaceandalignedsothat enablingcoincidencemeasurementsoffragment-γ andβ-
the segmentation in each successive detector alternated γ events. A coincidence register monitored signals from
5
the other Si detectors in the BCS telescope, reducing
dead time by signaling readout only for energy signals 84Mo
within the master gate.
s
1 100
/
s
III. RESULTS AND DISCUSSION t
n
u
o
C
The decay curve for β-decay events that occurred
within10sinthesamepixelor4nearest-neighborpixels
of a 84Mo implantation is shown in Fig. 6. These data 10
werefitbasedonthemaximizationofaPoissonprobabil- 0 2 4 6 8 10
ity log-likelihood function that considered the exponen- Time (s)
tialdecayofthe84Moparent,theexponentialgrowthand
decayofthe84NbdaughterusingaT of9.5s[25],and 83
1/2 Nb
alinear background. The deducedhalf-life was2.0±0.4 1000
s
sbasedonasamplesizerepresenting1037implantations.
1
The reliability of the analysis approach is demonstrated /
by the reproduction of the known half-lives of 83Nb and ts
n
81Zr. The half-life of 83Nb was deduced by Kuroyanagi u
o
et al. [27] as 4.1±0.3 s. The value deduced from the C
decay curve in Fig. 6 was 3.8±0.2 s, consistentwith the
previous result. A half-life of 5.3±0.5 s for 81Zr was ex- 100
tracted by Huang et al. [28, 29] from a fit of a selective
0 5 10 15 20
γp-gated decay curve. The fit of the decay curve for β
Time (s)
events correlated with 81Zr implantations shown in Fig.
6resultedinahalf-lifevalueof5.0±0.2s. Adecaycurve
for81Zrwasalsogeneratedbygatingonknownγ raysin 81
Zr
the daughter 81Y with energies 113, 175, and 230 keV,
s 1000
thatwereobservedfollowingβ decayforthefirsttime. A 1
fittothismoreselectivedecaycurve,takingintoaccount /
s
the exponential decay of the 81Zr parent and a constant nt
background,culminatedinahalf-life of4.8±0.5s,again u
o
consistent with the the previously reported half-life and C
thatdeducedhereconsideringonlyβ-decayeventscorre-
lated with 81Zr implantations. 100
One shortcoming of the fits performed with the Pois- 0 5 10 15 20 25
son probability log-likelihood function is that all decay Time (s)
times were considered to be independent, which does
notaccuratelydescribethe probabilitydensityofdaugh- 1000
ter decays. Therefore, decay events were also evaluated 81
Zr
using a Maximum Likelihood analysis (MLH) [30] that
s
γ gated
considered β-decay chains of up to 3 members. Analy- 1
sis of the 84Mo decay chain considered events that oc- /
s
curred within 20 s in the same pixel of a 84Mo implan- nt 100
tation. Fixed values of 9.5 s and 25.9 min were used for ou
the decay time parameters of the daughter and grand- C
daughter,respectively. Backgroundratesspecifictoeach
individual decay chain were determined by measuring 10
the average rate of uncorrelated β events on a pixel-by- 0 5 10 15 20 25
pixel basis. A half-life of 2.2 ± 0.2 s was deduced using Time (s)
all 366 extracted chains. This value is consistent with
the above described Poisson distribution maximum like-
lihood method, though the approachis inherently better FIG. 6: Decay curves for 84Mo, 83Nb, and 81Zr. The solid
and dashed lines are the parent and daughter contributions,
because it relies on a probability density function that
respectively, to the decay and include a linear background.
considers the dependencies of the parent, daughter, and
Details of thefitting procedureare given in thetext.
granddaughter decays, as well as background.
Thenewhalf-lifevalueismorethan1σbelowthevalue
of 3.7 +1.0 deduced from 37 implantations by Kienle et
−0.8
6
al. [18]. The new, shorter value continues the system- The implications of the newly measured84Mo half-life
atic trend observed in Table I relative to the previously on the rp-process were calculated using a single zone X-
adopted value. It is also consistent with the above dis- ray burst model based on ReaclibV1 rates provided by
cussed deformationassumed in the Sarrigurenet al. [12] JINA Reaclib online database [32]. The abundance with
self-consistenttreatmentandaffirmsthe trendtowardsa respect to burst duration is shown in Fig. 8 for 84Mo
spherical100Sn implied by the measured84Mo R [10]. (solidlines)andforallA=84isobars(dashedline). The
4/2
shaded region covers the range of previously predicted
The initial intent of the half-life remeasurement of
84Mo was to gain additional selectivity in the analysis half-lives (0.8 s ≤ T1/2 ≤ 6.0 s) given by various models
[12, 13, 33]. The dot-dashed line represents the yield
ofthedecaycurvebyγ-raygating. Theenergyspectrum
for γ rays found in coincidence with a 84Mo β decay is calculated using the experimental upper limit of 4.7 s
takenfromthepreviouslyadopted84MoT . Theorder
shown in Fig. 7. Unfortunately, no γ rays were observed 1/2
that could be attributed to the 84Nb daughter. Previ- of magnitude uncertainty in the final 84Sr abundance is
reduced to less than a factor 2 with the new half-life.
ousworkhasrestrictedtheground-statespinassignment
for 84Nb to 1, 2, or 3 with positive parity. A 1+ 84Nb
ground-statewouldbeconsistentwiththelowγ intensity
observedduringβ decayfromthe84Mo0+ ground-state.
Additionally, a 1+ assignment would allow direct transi-
tions into the the 0+ 84Zr ground state and explain the
lack of evidence in Fig. 7 for previously observed states
populated via 84Nb β decay [31]. Based on the simplis-
tic assumption that 100% of the 84Nb β-decays feed ex-
cited states in 84Zr that depopulate through the known
2+ level at 540.0 keV, we expected to observe a total of
1
14(9)countsatthisenergyinFig.7. Theabsenceofthis
540.0 keV transition is consistent with a portion of the
84Nbβ decaypopulatingthe84Zrgroundstate. Thisfur-
ther supports a 1+ spin and parity for the 84Nb ground
state.
FIG. 8: Impact of 84Mo half-life on the final A = 84 isobar
abundanceusingasinglezoneX-rayburstcalculation. Abun-
danceisreportedasaratioof(massfraction)/(massnumber).
Time 0corresponds tothehydrogen-ignition start time. The
solid line, bounded above and below with dotted uncertain-
ties,showstheresultusingthenewly-measured84Mohalflife
(T = 2.2±0.2 s). The dashed line corresponds tosummed
1/2
abundance of all A=84 isobars. The dot-dashed line repre-
sents theyield calculated using theexperimental upperlimit
of4.7stakenfromthepreviouslyadopted84MoT . Shaded
1/2
regions highlight the range in abundance based on half-lives
predicted previously (0.8 s≤T ≤6.0 s).
1/2
The amount of material processed above 56Ni can be-
comesignificantduringX-rayburststhatlastlongerthan
10s. Insuchascenario,theT of84Mocanhaveacon-
1/2
siderableimpactontheoverallmassprocessing. Aprevi-
ous burst simulation [7] based on a low α-separationen-
ergyfor84Moprovidedbythe1992FiniteRangeDroplet
Model predicted a Zr-Nb cycle via the reactionsequence
83Nb(p,α)80Zr(β+)80Y(p,γ)81Zr
FIG.7: Spectrumofγraysincoincidencewithβ-decayevents (β+)81Y(p,γ)
occurringinthesameandthe4nearest-neighborpixelswithin (cid:26)(p,γ)82Nb(β+)(cid:27) 82Zr(p,γ)83Nb.
10sofa84Moimplantation. Thetransitionwithenergy385.9
keVrepresentsthedecayofthe2+1 statein80Srfedfrom 80Y Escape from this cycle is only possible through 84Mo β-
β-decay,theprinciple source of background decay events. decay. The newly reported half-life is still longer than
7
the 1.1 s value used for simulations in [7], leading to a impact of 84Mo on the rp-process mass flow. γ-ray data
more pronounced bottleneck than predicted. The estab- werenot sufficient to restrictthe 84Nb ground-statespin
lishment of a Zr-Nb cycle would make the impact of the beyondwhatwaspreviouslyreported,thoughatentative
84Mo T for somerp-processscenariosconsiderablein- 1+ assignment is favored.
1/2
deed. Measuringtheα-separationenergyof84Moiscrit- The experiment discussed in this paper was the first
icalto determiningthe existence ofsucha cycle,andthe to use the NSCL RFFS. The TOF-specific beam rejec-
realimpactthatithasonmassprocessingaboveA=84. tioncapability,criticalforselectivelyreducingtheoverall
beamrate,isshowntobesufficienttoenablefragment-β
correlationof neutron-deficient, longer-lived species.
IV. CONCLUSION
The half-life for the rp-process waiting point nucleus Acknowledgments
84Mo has been re-measuredto be 2.2 ± 0.2 s, more than
1σ shorter than the previously adopted value. This new The authors thank the NSCL operations staff for pro-
value is in line with the theoretical predictions of Sar- viding the primary and secondarybeams for this experi-
riguren et al. of the mid-mass N = Z region consistent mentandNSCLγ groupforassistancesettinguptheGe
with a 84Mo nucleus that begins a shape transition to- detectors from SeGA. We are appreciative of insightful
wards a spherical 100Sn. 84Mo is an important waiting statistics discussions with G.F. Grinyer and J. Thorpe.
point in the rp-process, determining mass abundance at This work was supported in part by the National Sci-
and procession above A = 84. A measurement of the ence Foundation Grant Nos. PHY-06-06007 and PHY-
84Moα-separationenergyiscriticaltodeterminethefull 05-20930and JINA grant PHY-02-16783.
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