Theoretical models of β-delayed neutron emission are used as crucial inputs in r-process calculations. Benchmarking the predictions of these models is a challenge due to a lack of currently available experimental data. In this work the β-delayed neutron emission probabilities of 33 nuclides in the important mass regions south and south-west of $^{132}$Sn are presented, 16 for the first time. The measurements were performed at RIKEN using the Advanced Implantation Detector Array (AIDA) and the BRIKEN neutron detector array. The <math><msub><mrow><mi>P</mi></mrow><mrow><mn>1n</mn></mrow></msub></math> values presented constrain the predictions of theoretical models in the region, affecting the final abundance distribution of the second r-process peak at <math><mi>A</mi><mo>≈</mo><mn>130</mn></math>.

United Kingdom

O. Hall

T. Davinson

United States

A. Estrade

Japan

Hong Kong

J. Liu

G. Lorusso

F. Montes

S. Nishimura

Vietnam

V.H. Phong

P.J. Woods

Spain

J. Agramunt

South Korea

D.S. Ahn

A. Algora

J.M. Allmond

H. Baba

S. Bae

N.T. Brewer

C.G. Bruno

Canada

R. Caballero-Folch

F. Calviño

P.J. Coleman-Smith

G. Cortes

I. Dillmann

C. Domingo-Pardo

Poland

A. Fijalkowska

N. Fukuda

S. Go

C.J. Griffin

R. Grzywacz

J. Ha

L.J. Harkness-Brennan

T. Isobe

D. Kahl

L.H. Khiem

Hungary

G.G. Kiss

A. Korgul

S. Kubono

M. Labiche

I. Lazarus

J. Liang

China

Z. Liu

K. Matsui

K. Miernik

B. Moon

A.I. Morales

P. Morrall

M.R. Mumpower

N. Nepal

R.D. Page

M. Piersa

V.F.E. Pucknell

B.C. Rasco

B. Rubio

K.P. Rykaczewski

H. Sakurai

Y. Shimizu

D.W. Stracener

T. Sumikama

H. Suzuki

J.L. Tain

H. Takeda

A. Tarifeño-Saldivia

A. Tolosa-Delgado

M. Wolińska-Cichocka

R. Yokoyama

-delayed Neutron Emission of -process Nuclei at the = 82 Shell Closure

The observation of oxygen isotopes in giant stars sheds light on mixing processes operating in their interiors. Due to the very strong correlation between nuclear burning and mixing processes it is very important to reduce the uncertainty on the cross sections of the nuclear reactions that are involved. In this paper we focus our attention on the reaction <math><mmultiscripts><mrow><mi>O</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>18</mn></mrow></mmultiscripts><msup><mrow><mo>(</mo><mi>p</mi><mo>,</mo><mi>γ</mi><mo>)</mo></mrow><mrow><mn>19</mn></mrow></msup><mi>F</mi></math>. While the <math><mmultiscripts><mrow><mi>O</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>18</mn></mrow></mmultiscripts><msup><mrow><mo>(</mo><mi>p</mi><mo>,</mo><mi>α</mi><mo>)</mo></mrow><mrow><mn>15</mn></mrow></msup><mi>N</mi></math> channel is thought to be dominant, the (p,γ) channel can still be an important component in stellar burning in giants, depending on the low energy cross section. So far only extrapolations from higher-energy measurements exist and recent estimates vary by orders of magnitude. These large uncertainties call for an experimental reinvestigation of this reaction. We present a direct measurement of the <math><mmultiscripts><mrow><mi>O</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>18</mn></mrow></mmultiscripts><msup><mrow><mo>(</mo><mi>p</mi><mo>,</mo><mi>γ</mi><mo>)</mo></mrow><mrow><mn>19</mn></mrow></msup><mi>F</mi></math> cross section using a high-efficiency 4π BGO summing detector at the Laboratory for Underground Nuclear Astrophysics (LUNA). The reaction cross section has been directly determined for the first time from 140 keV down to 85 keV and the different cross section components have been obtained individually. The previously highly uncertain strength of the 90 keV resonance was found to be 0.53 ± 0.07 neV, three orders of magnitude lower than an indirect estimate based on nuclear properties of the resonant state and a factor of 20 lower than a recently established upper limit, excluding the possibility that the 90 keV resonance can contribute significantly to the stellar reaction rate.

Italy

A. Best

F.R. Pantaleo

A. Boeltzig

G. Imbriani

M. Aliotta

J. Balibrea-Correa

Germany

D. Bemmerer

C. Broggini

R. Buompane

A. Caciolli

F. Cavanna

T. Chillery

G.F. Ciani

P. Corvisiero

L. Csedreki

R.J. deBoer

R. Depalo

A. Di Leva

Z. Elekes

F. Ferraro

E.M. Fiore

A. Formicola

Zs. Fülöp

G. Gervino

A. Guglielmetti

C. Gustavino

Gy. Gyürky

M. Junker

I. Kochanek

M. Lugaro

P. Marigo

R. Menegazzo

V. Mossa

V. Paticchio

R. Perrino

D. Piatti

P. Prati

L. Schiavulli

K. Stöckel

O. Straniero

F. Strieder

T. Szücs

M.P. Takács

D. Trezzi

M. Wiescher

S. Zavatarelli

Cross section of the reaction O(p, ) F at astrophysical energies: The 90 keV resonance and the direct capture component

The NeNa and the MgAl cycles play a fundamental role in the nucleosynthesis of asymptotic giant branch stars undergoing hot bottom burning. The <math><mmultiscripts><mrow><mtext>Na</mtext></mrow><mprescripts></mprescripts><none></none><mrow><mn>23</mn></mrow></mmultiscripts><mo>(</mo><mtext>p</mtext><mo>,</mo><mi>γ</mi><mo>)</mo><msup><mrow></mrow><mrow><mn>24</mn></mrow></msup><mtext>Mg</mtext></math> reaction links these two cycles and a precise determination of its rate is required to correctly estimate the contribution of these stars to the chemical evolution of various isotopes of Na, Mg and Al. At temperatures of <math><mn>50</mn><mo>≲</mo><mi>T</mi><mo>≲</mo><mn>110</mn><mspace width="0.2em"></mspace><mtext>MK</mtext></math>, narrow resonances at <math><msub><mrow><mi>E</mi></mrow><mrow><mi>p</mi></mrow></msub><mo>=</mo><mn>140</mn></math> and <math><mn>251</mn><mspace width="0.2em"></mspace><mtext>keV</mtext></math> are the main contributors to the reaction rate, in addition to the direct capture that dominates in the lower part of the temperature range. We present new measurements of the strengths of these resonances at the Laboratory for Underground Nuclear Astrophysics (LUNA). We have used two complementary detection approaches: high efficiency with a 4π BGO detector for the 140keV resonance, and high resolution with a HPGe detector for the 251keV resonance. Thanks to the reduced cosmic ray background of LUNA, we were able to determine the resonance strength of the 251keV resonance as <math><mi>ω</mi><mi>γ</mi><mo>=</mo><mn>482</mn><mo>(</mo><mn>82</mn><mo>)</mo><mspace width="0.2em"></mspace><mrow><mi>μ</mi><mtext>eV</mtext></mrow></math> and observed new gamma ray transitions for the decay of the corresponding state in <math><mmultiscripts><mrow><mtext>Mg</mtext></mrow><mprescripts></mprescripts><none></none><mrow><mn>24</mn></mrow></mmultiscripts></math> at <math><msub><mrow><mi>E</mi></mrow><mrow><mi>x</mi></mrow></msub><mo>=</mo><mn>11931</mn><mspace width="0.2em"></mspace><mtext>keV</mtext></math>. With the highly efficient BGO detector, we observed a signal for the 140keV resonance for the first time in a direct measurement, resulting in a strength of <math><mi>ω</mi><msub><mrow><mi>γ</mi></mrow><mrow><mn>140</mn></mrow></msub><mo>=</mo><msubsup><mrow><mn>1.46</mn></mrow><mrow><mo>−</mo><mn>0.53</mn></mrow><mrow><mo>+</mo><mn>0.58</mn></mrow></msubsup><mspace width="0.2em"></mspace><mrow><mi>neV</mi></mrow></math> (68% CL). Our measurement reduces the uncertainty of the <math><mmultiscripts><mrow><mtext>Na</mtext></mrow><mprescripts></mprescripts><none></none><mrow><mn>23</mn></mrow></mmultiscripts><mo>(</mo><mtext>p</mtext><mo>,</mo><mi>γ</mi><mo>)</mo><msup><mrow></mrow><mrow><mn>24</mn></mrow></msup><mtext>Mg</mtext></math> reaction rate in the temperature range from 0.05 to 0.1GK to at most <math><msubsup><mrow></mrow><mrow><mo>−</mo><mn>35</mn><mtext>%</mtext></mrow><mrow><mo>+</mo><mn>50</mn><mtext>%</mtext></mrow></msubsup></math> at 0.07GK. Accordingly, our results imply a significant reduction of the uncertainties in the nucleosynthesis calculations.

F. Munnik

Direct measurements of low-energy resonance strengths of the Na(p, ) Mg reaction for astrophysics

The $^{18}$O(p,α)$^{15}$N reaction affects the synthesis of $^{15}$N, $^{18}$O and $^{19}$F isotopes, whose abundances can be used to probe the nucleosynthesis and mixing processes occurring deep inside asymptotic giant branch (AGB) stars. We performed a low-background direct measurement of the $^{18}$O(p,α)$^{15}$N reaction cross-section at the Laboratory for Underground Nuclear Astrophysics (LUNA) from center of mass energy <math><msub><mrow><mi>E</mi></mrow><mrow><mtext>c.m.</mtext></mrow></msub><mo>=</mo><mn>340</mn></math> keV down to <math><msub><mrow><mi>E</mi></mrow><mrow><mtext>c.m.</mtext></mrow></msub><mo>=</mo><mn>55</mn></math> keV, the lowest energy measured to date corresponding to a cross-section of less than 1 picobarn/sr. The strength of a key resonance at center of mass energy <math><msub><mrow><mi>E</mi></mrow><mrow><mi>r</mi></mrow></msub><mo>=</mo><mn>90</mn></math> keV was found to be a factor of 10 higher than previously reported. A multi-channel R-matrix analysis of our and other data available in the literature was performed. Over a wide temperature range, <math><mi>T</mi><mo>=</mo><mn>0.01</mn><mtext>–</mtext><mn>1.00</mn></math> GK, our new astrophysical rate is both more accurate and precise than recent evaluations. Stronger constraints can now be placed on the physical processes controlling nucleosynthesis in AGB stars with interesting consequences on the abundance of $^{18}$O in these stars and in stardust grains, specifically on the production sites of oxygen-rich Group II grains.

Belgium

P. Descouvemont

Improved astrophysical rate for the O(p, ) N reaction by underground measurements