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astrophysical models can explain the origin of most nuclei beyond the iron group in a combination of processes involving neutron captures on long ( _ s _ process ) or short ( _ r _ process ) time scales @xcite .
however , 32 stable , proton - rich isotopes between @xmath2se and @xmath10hg can not be formed in that way . those _ p _
nuclei are 10 to 100 times less abundant than the _ s _ and _
r _ nuclei in the same mass region .
they are thought to be produced in the so - called @xmath0 or _
process , where proton - rich nuclei are made by sequences of photodisintegrations and @xmath11 decays @xcite . in this scenario ,
pre - existing seed nuclei from the _ s _ and _ r _ processes are destroyed by photodisintegration in a high - temperature environment , and proton - rich isotopes are produced by ( @xmath0__,n _ _ ) reactions .
when ( @xmath0__,p _ _ ) and ( @xmath12 ) reactions become comparable or faster than neutron emission within an isotopic chain , the reaction path branches out and feeds nuclei with lower charge number _
z_. the decrease in temperature at later stages of the _ p _ process leads to a freeze - out via neutron captures and mainly @xmath11 decays , resulting in the typical _
p_-process abundance pattern with maxima at @xmath13mo ( _ _ n__=50 ) and @xmath14sm ( _ _ n__=82 ) .
the currently most favored astrophysical site for the _ p _ process is explosive burning in type ii supernovae .
the explosive shock front heats the outer o / ne shell of the progenitor star to temperatures of 2 - 3 gk , sufficient for providing the required photodisintegrations . following the nucleosynthesis in such astrophysical models , good agreement with the required _
production is found , with exception of the low ( _ _ a__@xmath4100 ) and intermediate ( 150@xmath15__a__@xmath15 165 ) mass range , which are underproduced by factors of 3 - 4 @xcite .
because of these persisting problems , alternative scenarios ( such as type ia supernovae and x - ray bursters ) have been suggested , each with their own , inherent difficulties @xcite . currently , however , it is not yet clear whether the observed underproductions are due to a problem with astrophysical models or with the nuclear physics input , i.e. reaction rates .
thus , a necessary requirement towards a consistent understanding of the _ p _ process is the reduction of uncertainties in nuclear data . by far most of the several hundreds of required photodisintegration rates and their inverses
need to be inferred from hauser - feshbach statistical model calculations @xcite , e.g. the codes non - smoker @xcite and most @xcite .
experimental data can improve the situation in two ways , either by directly replacing predictions with measured cross sections in the relevant energy range , or by testing the reliability of predictions at other energies when the relevant energy range is not experimentally accessible .    the role of ( _ n_,@xmath0 ) reactions in the _ p _ process was underestimated for a long time , although it is obvious that they have an influence on the final _ p_-process abundances .
neutron captures compete with ( @xmath0,_n _ ) reactions and thus hinder the photodisintegration flux towards light nuclei , especially at lower-_z _ isotopes and even - even isotopes in the vicinity of branching - points .
the influence of a variation of reaction rates on the final _ p _ abundances has been studied previously @xcite .
it turned out that the _ p _ abundances are very sensitive to changes of the neutron - induced rates in the entire mass range , whereas the proton - induced and @xmath16-induced reaction rates are important at low and high mass numbers , respectively .
rayet et al .
@xcite have also studied the influence of several components in their _
p_-process network calculations .
their nuclear flow schemes show that branching points occur even at light _
nuclei , and are shifted deeper into the proton - rich unstable region with increasing mass and temperature . in contradiction to woosley and howard @xcite , who claimed for their network calculations that ( _ n_,@xmath0 ) can be neglected except for the lightest nuclei ( _ _ a__@xmath1590 ) , rayet et al . also examined the influence of neutron reactions for temperatures between _ _
t__@xmath17= 2.2 and 3.2 gk by comparing overabundance factors if ( _ n_,@xmath0 ) reactions on _ _ z__@xmath626 nuclides are considered or completely suppressed . as a result , the overabundances were found to change by up to a factor 100 ( for @xmath3sr ) if the ( _ n_,@xmath0 ) channel was artificially suppressed .
this rather high sensitivity indicates the need for reliable ( _ n_,@xmath0 ) rates to be used in _
p_-process networks .
although recent efforts are directed to calculation or measurement of photodisintegration cross sections and rates @xcite , astrophysical photodisintegration rates can easily be inferred from capture rates by detailed balance , even in theoretical work @xcite .
the stellar reaction rate n@xmath18@xmath4@xmath19@xmath6@xmath20@xmath21 for the reaction is related to its inverse rate by @xmath22 with the avogadro number _ n@xmath18 _ , the nuclear spins _
j _ and masses _ a _ , the respective temperature - dependent partition functions _
g(t ) _ and the reaction _ q _ value in the exponent .
measuring or calculating a rate in the direction of positive _ q _ value ensures best numerical accuracy and consistency between forward and backward reaction .
this is important when implementing those rates in reaction networks for nucleosynthesis models .    moreover , stellar cross sections and rates have to be employed for the computation of reverse rates . in a stellar environment
, nuclei are fully thermalized with the environment , resulting in a thermal excitation of both the target and the final nucleus .
only stellar cross sections including the excitation in form of a stellar enhancement factor ( sef ) can be used to properly account for all transitions when applying detailed balance . for reactions with positive _ q _
values for captures , a laboratory measurement of the capture cross section will encompass by far more of the relevant transitions than a photodisintegration experiment , even with the target being in the ground state @xcite .    for the past decade
there has been a continuing effort to measure nuclear data for the _ p _ process , both for charged particle reactions @xcite and for neutron induced reactions @xcite .
the present work comprises the first measurement of ( _ n_,@xmath0 ) cross sections for the _ p_-process isotopes @xmath2se and @xmath3sr at _ _
kt__= 25 kev , with the aim to improve the _
p_-process database and to help testing theoretical predictions . since it is not possible to measure cross sections directly at _
p_-process temperatures of _
_ kt__= 170 - 260 kev , we have to perform the measurements at _ s_-process ( and freeze - out ) temperatures of _ _ kt__= 25 kev , and then extrapolate theoretically by means of the respective energy dependent cross sections ( see sec .  [
theory ] ) .
the measurement of stellar ( _ n_,@xmath0 ) rates requires a `` stellar '' neutron source yielding neutrons with a maxwell - boltzmann energy distribution .
we achieve this by making use of the @xmath1li(_p , n_)@xmath1be reaction . in combination with the activation or time - of - flight technique ,
this offers a unique tool for comprehensive studies of ( _ n_,@xmath0 ) rates and cross sections for astrophysics .    in sec .
[ exp ] , the experimental technique and sample characteristics are outlined , followed by the description of the data analysis in sec .  [ data ] .
the results are presented in sec .
a comparison to theory and extrapolation to higher energies is given in sec .
[ theory ] .
the paper is concluded with a summary and a short outlook in sec .
all measurements were carried out at the karlsruhe 3.7 mv van de graaff using the activation technique .
neutrons were produced with the @xmath1li(_p , n_)@xmath1be source by bombarding 30 @xmath23 m thick layers of metallic li on a water - cooled cu backing with protons of 1912 kev , 30 kev above the reaction threshold .
the resulting quasi - stellar neutron spectrum approximates a maxwellian distribution for _ _
kt__= @xcite .
hence , the proper stellar capture cross section can be directly deduced from our measurement .    for the activations natural samples of selenium metal ( 0.89% @xmath2se ) and various strontium compounds ( 0.56% @xmath3sr ) were used . in order to verify the stoichiometry ,
samples of sr(oh)@xmath24 and srf@xmath24 were dried at 300@xmath25c and 800@xmath25c , respectively .
the powders were pressed to thin pellets , which were enclosed in a 15 @xmath23 m thick aluminium foil and sandwiched between 10 - 30 @xmath23 m thick gold foils of the same diameter . in this way the neutron flux can be determined relative to the well - known capture cross section of @xmath26au @xcite .
the activation measurements were carried out with the van de graaff accelerator operated in dc mode with a current of @xmath27100  @xmath23a .
the mean neutron flux over the period of the activations was @xmath271.5@xmath2810@xmath29 _
n_/s at the position of the samples , which were placed in close geometry to the li target . throughout the irradiation
the neutron flux was recorded in intervals of 1  min using a @xmath30li - glass detector for later correction of the number of nuclei , which decayed during the activation ( factor f@xmath31 in eq .
[ eq:4 ] ) .    over the course of the present measurements ,
a total of 17 activations ( 5 for se and 12 for sr ) have been carried out with modified experimental parameters ( see table  [ tab : table1 ] ) .
five short - time activations of 3  h to 5  h were used for determining the partial cross section of the @xmath3sr(_n_,@xmath0)@xmath9sr@xmath32 reaction feeding the isomer in @xmath9sr with a half - life of 67.6 m. the @xmath3sr(_n_,@xmath0)@xmath9sr@xmath33 cross section to the ground state was separately deduced from seven long - time activations .
.[tab : table1]activation schemes and sample characteristics .
the suffix `` m '' denotes short time activations for measurements of the partial cross section to the @xmath9sr@xmath34 isomeric state .
@xmath35@xmath36 gives the neutron exposure of the sample during the activation .
[ cols="<,^,^,^,^,^ " , ]
we have presented the first results of an ongoing experimental program to determine more precise _
p_-process reaction rates in the mass range _ _ a__=70 - 140 .
the stellar ( _ n_,@xmath0 ) cross sections of the _ p _ nuclei @xmath2se and @xmath3sr have been measured for the first time , yielding values of 281  mb for @xmath2se , and 112  mb for the ground and 189  mb for the isomeric state to @xmath9sr with our experimental neutron spectrum .
the respective maxwellian averaged cross sections for _ _
kt__= 30 kev were derived with the energy dependence of jeff 3.0 and result in @xmath4@xmath5@xmath6@xmath7= 271@xmath815  mb for @xmath2se , and @xmath4@xmath5@xmath6@xmath7(total)= 300 @xmath817  mb for @xmath3sr .
the isomeric ratio ir was found to be 0.63 @xmath8 0.04 and thus yields a partial stellar cross section of @xmath4@xmath5@xmath6@xmath7(part.)= 190@xmath810 mb .    over the past decade , a lot of work has been devoted to measure cross sections and reaction rates of _ p _ nuclei , but experimental ( _ p_,@xmath0 ) , ( @xmath16,@xmath0 ) and photodisintegration rates are still very scarce .
the situation for stellar ( _ n_,@xmath0 ) cross sections is somewhat better , but it should be pointed out that nearly all of the ( _ n_,@xmath0 ) measurements were performed in energy regions relevant for the _ s _ process ( _ _ kt__= 30 kev instead of 100@xmath4__kt__@xmath4260 kev for the _ p _ process ) , whereas the charged particle rates are measured close to the respective _ p_-process gamow window .
the measurements presented in this paper mark the beginning of an extensive experimental program to determine more precise neutron cross sections of stable _ p _ nuclei . within this program ,
we have already finished the measurement on @xmath37ru @xcite , and preliminary values are available for @xmath38pd , @xmath39te , @xmath40ba , @xmath41ba and @xmath42hf @xcite .
all available experimental information will be summarized in an upcoming paper , including an extrapolation to the full range of _
p_-process temperatures and the calculation of inverse reaction rates by detailed balance .
we thank e. p. knaetsch , d. roller and w. seith for their help and support during the irradiations at the van de graaff accelerator .
this work was supported by the swiss national science foundation grants 2024 - 067428.01 and 2000 - 105328 .
the nucleosynthesis of elements beyond iron is dominated by the _ s _ and _ r _ processes . 
 however , a small amount of stable isotopes on the proton - rich side can not be made by neutron capture and are thought to be produced by photodisintegration reactions on existing seed nuclei in the so - called `` _ _ p _ _ process '' . 
 so far most of the _ 
 p_-process reactions are not yet accessible by experimental techniques and have to be inferred from statistical hauser - feshbach model calculations . 
 the parametrization of these models has to be constrained by measurements on stable proton - rich nuclei . a series of ( _ n_,@xmath0 ) activation measurements , related by detailed balance to the respective photodisintegrations , 
 were carried out at the karlsruhe van de graaff accelerator using the @xmath1li(_p , n_)@xmath1be source for simulating a maxwellian neutron distribution of _ _ 
 kt__= 25 kev . 
 first results for the experimental ( _ n_,@xmath0 ) cross sections of the light _ p _ nuclei @xmath2se and @xmath3sr are reported . 
 these experimental values were used for an extrapolation to the maxwellian averaged cross section at 30 kev , @xmath4@xmath5@xmath6@xmath7 , yielding 271@xmath815  mb for @xmath2se , and 300@xmath817  mb for the total capture cross section of @xmath3sr . the partial cross section to the isomer in @xmath9sr was found to be 190@xmath810  mb .