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the soudan-2 experiment is currently taking data using its fine - grained iron tracking calorimeter of total mass 963 tons . this detector images non - relativistic as well as relativistic charged particles produced in atmospheric neutrino reactions . it is operating underground at a depth of 2100 meters - water - equivalent on level 27 of the soudan mine state park in northern minnesota ( northwest of sudbury ) . the calorimeter s modular design enabled data - taking to commence in april 1989 when the detector was one quarter of its full size ; assembly of the detector was completed during 1993 . data - taking has continued with 85% live time , even though dynamite blasting has been underway nearby for the minos cavern excavation since summer 1999 . the data exposure as of this conference is 5.40 fiducial kiloton - years ( kty ) . results presented here are based upon a 5.1 kty exposure . the tracking calorimeter operates as a slow - drift ( 0.6 cm/@xmath12s ) time projection chamber . its tracking elements are meter - long plastic drift tubes which are placed into the corrugations of steel sheets . the sheets are stacked to form a tracking lattice of honeycomb geometry . a stack is packaged as a calorimeter module and the detector is assembled building - block fashion using these modules  @xcite . the calorimeter is surrounded on all sides by a cavern - liner active shield array of two or three layers of proportional tubes  @xcite . topologies for contained events in soudan 2 include single track and single shower events ( mostly @xmath0  and @xmath1  quasi - elastic reactions ) and multiprong events . flavor - tagging proceeds straightforwardly : an event having a leading , non - scattering track with ionization @xmath13 compatible with muon mass is a candidate charged current ( cc ) event of ; an event having a prompt , relatively energetic shower prong is a candidate @xmath1 cc event . recoil protons of momenta greater than approx . 350 mev/@xmath14 are imaged by the calorimeter , allowing a much - improved measurement of the incident neutrino direction , especially for sub - gev quasi - elastic reactions . we measure the atmospheric neutrino @xmath0/@xmath1  flavor ratio - of - ratios @xmath16 using single track and single shower events which are fully contained within the calorimeter ( all hits more than 20 cm from the nearest surface ) . these samples contain mostly quasi - elastic neutrino reactions , but include a background of photon and neutron reactions originating in cosmic ray muon interactions in the surrounding cavern rock . the latter  rock events " are mostly tagged by coincident hits in the active shield , however some are unaccompanied by shield hits and constitute a background . the amount of zero - shield - hit rock background in a neutrino event sample is estimated by fitting event vertex - depth distributions to a combination of tagged - rock and @xmath15 monte carlo distributions . as expected , the fits show the background to be mostly confined to outer regions of the calorimeter . details of our analysis procedures for quasi - elastic events can be found in refs . @xcite . the track and shower event samples for our 5.1 kty exposure are summarized in table  [ tab : results ] . our full detector monte carlo simulation of atmospheric neutrino interactions is based on the 1996 bartol flux for the soudan site  @xcite . .soudan-2 track and shower event samples from the 5.1 fiducial kiloton - year exposure . [ cols="<,^,^",options="header " , ]     [ tab : hires ]    the atmospheric monte carlo ( mc ) sample represents 28.2 kiloton years of exposure . the mc event rates displayed in table  [ tab : hires ] have been normalized to the @xmath1  data sample . this normalization is equivalent to a reduction of the bartol neutrino fluxes by 21% . the assumption implicit with this adjustment is that the @xmath1   sample is devoid of oscillation effects [ fig : fig01 ] ,  [ fig : fig02 ]  and  [ fig : fig03 ] show hires distributions with this normalization in place .    fig . [ fig : fig01 ] shows the distributions of these samples in cosine of the zenith angle . for @xmath1  events , the shape of the distribution for data ( fig . [ fig : fig01]a , crosses ) coincides with that predicted by the monte carlo ( dashed histogram ) for null oscillations . the distribution of @xmath0  data however , falls below the mc prediction in all bins ( fig . [ fig : fig01]b ) with the relative dearth being more pronounced for @xmath0 s incident from below horizon . distributions in @xmath17 for hires events are shown in fig . [ fig : fig02 ] . for null oscillations this variable distributes according to a ` phase space ' which reflects the neutrino points - of - origin throughout the spherical shell volume of the earth s atmosphere . that is , down - going @xmath15 s populate the peak at lower @xmath17 from 0.0 to 2.0 . neutrinos incident from / near the horizon occur within the dip region extending from 2.0 to 2.6 , while upward - going neutrinos populate the peak at higher values . [ fig : fig02]a shows that , allowing for statistical fluctuations , the @xmath1  data follows the shape of the null oscillation mc distribution . in contrast , the @xmath0  data ( fig . [ fig : fig02]b ) falls below the null ocillation mc for all but the most vertically down - going flux . to convert results of our atmospheric neutrino simulation generated under the no - oscillation hypothesis into simulated neutrino oscillation data , we apply to every mc event an @xmath4-dependent weight representing the probability of  survival for a given @xmath8  and @xmath7 .    an exploratory matchup of @xmath0  data with trial oscillation scenarios is shown in fig . [ fig : fig03 ] . for the mixing angle @xmath7  fixed at unity , we plot the mc distribution weighted for @xmath0  to @xmath3  oscillations with differing @xmath8  settings . with @xmath18 ( fig . [ fig : fig03]a ) , the oscillation prediction lies above the data in most @xmath4 bins . with @xmath19 ( fig . [ fig : fig03]b ) , the upgoing @xmath15 flux is now better described by the mc , although the expectation for horizontal and down - going neutrinos remains somewhat high . with @xmath20 , a rough agreement overall is achieved ( fig . [ fig : fig03]c ) . going to higher @xmath8 , we find that for @xmath21 the oscillation - weighted mc falls below the data in most bins ( fig . [ fig : fig03]d ) . these hints concerning the parameters regime preferred by the data are borne out by a more considered analysis , as we now show .    to determine the neutrino oscillation parameters @xmath8  and @xmath7  from our data , we construct a @xmath22  function over the plane - of - parameters . for points @xmath23 in the physical region of the @xmath24 plane , we fit the mc expection to our data at each point . the mc flux normalization , @xmath25 as well as @xmath26 and @xmath27 , is a free parameter : @xmath28 we assume that the oscillation affecting our data is purely @xmath0  into @xmath3  and that the @xmath1  data is unaffected .    the @xmath22  is summed over data bins containing our selected ( hires ) @xmath0  and @xmath1  samples , where @xmath29 are @xmath0   @xmath17 bins , with @xmath30 containing all the @xmath1  events . the denominator @xmath31 accounts for finite statistics in the neutrino monte carlo and for uncertainty in the rock background in the @xmath15 data . not yet included are error terms which address systematic errors in the analysis , however preliminary examination shows statistical errors to be the dominant error source in the analysis . the mc counts @xmath32 for the @xmath33 bin are constructed using oscillation weight factors . we find the location of minimum @xmath34 , and plot @xmath35 which is @xmath36 . the @xmath37 surface thereby obtained is shown in fig . [ fig : fig04 ] . a crater region of low @xmath22 values is clearly discerned , at the bottom of which is a relatively flat basin . the lowest point @xmath38 occurs at values @xmath7  = 0.90 , @xmath39 , with flux normalization @xmath25 = 0.78 . an additional structure is the @xmath37 ridge which occurs at large mixing angle and for @xmath8  above 10@xmath40 ev@xmath41 . for oscillation solutions in this regime , depletion in the downward - going @xmath0  neutrino flux with sub - gev energies is predicted for @xmath0  to @xmath3  oscillations by equation ( 1 ) arising from the first oscillation minimum . our hires events have sufficient resolution to show such an effect if it would be present . however , no pronounced depletion is observed , and so the @xmath22  has a high value there .    to find the region allowed for the oscillation parameters by our data at 90% confidence level ( cl ) , we use the method of feldman and cousins  @xcite . at each of 2500 points @xmath42 on a grid spanning the physical region of the plane parameters , we run 1000 simulated experiments . for each of the simulated sets , we find @xmath43 such that @xmath44 is less than @xmath43 for 90% of the simulated experiments at @xmath23 . the surface defined by local @xmath45 over the oscillation parameters plane is shown in fig . [ fig : fig05 ] . note that the surface is not a plane at @xmath45 = 4.61 , but rather has a concave shape . the central shaded portion is approximately @xmath37 = 4.6 , however the outlying regions have @xmath37 values which are lower . at each point over the physical region , if @xmath46 is less than @xmath47 , then @xmath23 belongs to the allowed region of the 90% cl contour . the region allowed by our data at 90% cl is shown by the shaded area in fig . [ fig : fig06 ] . although @xmath38 occurs at the location depicted by the solid circle , the relatively flat basin of our @xmath37 surface extends to lower @xmath8  values . superk has reported their best fit @xmath8  value to be  @xcite ; our data is compatible with that as well as with somewhat higher @xmath8  values . we plan to include more data in the above analysis . an additional data sample is comprised of @xmath0  flavor events which are partially contained . for each event of this category , the primary vertex is required to be @xmath48 80 cm ( one hadronic interaction length ) from exterior surfaces of the calorimeter , and the final state must contain a non - scattering , exiting track with ionization compatible with a muon mass . this sample is useful because the assignment of @xmath0  flavor is reliable to better than 98% , and because the events are relatively energetic and consequently  point well " to the incident @xmath15 direction . the mean energy for neutrinos which initiate pces is estimated to be 4.7 gev , to be compared to a mean energy of 1.3 gev for @xmath0  hires events . the mean angular deviation of the reconstructed @xmath0  direction versus the true direction ( in monte carlo ) is @xmath49 . unfortunately the number of @xmath0  pces is low , less than one - third the population of our @xmath0  hires sample .    in order to isolate pce two - prong and multiprong topologies , the data events ( with mc events interspersed throughout ) are processed through a software filter ; those which pass are scanned . the filter is designed to eliminate downward - stopping muons which have endpoint decays . in soudan-2 , an electron from a muon decay near rangeout can give rise to a small shower of @xmath50 10 hits from the end of a muon track ; the topology is roughly akin to that of a neutrino - induced two - prong . consequently care must be taken to avoid remnant up - down asymmetry in pces introduced by the filter . the problem is avoided by requiring that there be @xmath48 20 hits from a pce vertex which are additional to the muon track .    to mitigate against cosmic - ray induced backgrounds we require that any hit in the active shield which is coincident with a pce , must be clearly associated with the exiting muon track . occasionally it happens that a charged pion ejected from the cavern rock is incident upon the calorimeter . if the pion penetrates by more than an interaction length and then scatters inelastically , it can mimic a @xmath15 pce topology . background events of this type are removed by requiring the net momentum of the hadronic system of a pce to lie within the same hemisphere which contains the candidate muon track .    with these selections we obtain 31 @xmath0  events for which the cosmic ray induced background is less than one event . the @xmath0  pce rate predicted by our monte carlo with no oscillations is 40 events . the distribution of pce data in @xmath51 can be compared , as done previously for contained hires @xmath0  events , to representative oscillation scenarios having @xmath7  = 1.0 . in fig . [ fig : fig07 ] we observe that , compared to the data ( crosses ) the oscillation prediction ( dashed histogram ) is relatively high for @xmath18 ( fig . [ fig : fig07]a ) . however the prediction for @xmath21 ( fig . [ fig : fig07]d ) is too low relative to the data . figs . [ fig : fig07]b , c suggest that the scenario preferred by the data lies in the regime between @xmath52 . in the near future , we will include the partially contained @xmath0  events into our @xmath22  fit to @xmath53 . additionally , a sample of upward - stopping muon events initiated by neutrino reactions below the detector has been isolated and will be analyzed for oscillation effects . and of course we will continue to accumulate and analyze new data . our goal is to keep the soudan-2 detector running and tuned for the change - of - beams incident at our underground site . the change will be from atmospheric @xmath15 s to fermilab @xmath0 s in fall 2003 , at which time the detector will serve the minos experiment  @xcite . mann , plenary talk at the xix int . symposium on lepton and photon interactions at high energies , stanford university , august 1999 , hep - ex/9912007 ; t. kafka , _ in : _ taup99 - proceedings of the sixth int . workshop on topics in astroparticle and underground physics , college de france , paris , france , september 1999 , nucl . b ( proc . suppl . ) * 87 * ( 2000 ) 186 .
neutrino interactions recorded in a 5.1 fiducial kiloton - year exposure of the soudan-2 iron tracking calorimeter are analyzed for effects of neutrino oscillations . 
 using contained single track and single shower events , we update our measurement of the atmospheric @xmath0/@xmath1  ratio - of - ratios and find @xmath2 . assuming this anomalously low r - value is the result of @xmath0  flavor disappearance via @xmath0  to @xmath3  oscillation , we select samples of charged current events which offer good resolution , event - by - event , for @xmath4  reconstruction . 
 oscillation - weighted monte carlo events are fitted to these data events using a @xmath5 function summed over bins of log@xmath6 . 
 the region allowed in the ( @xmath7 , @xmath8 ) plane at 90% cl is obtained using the feldman - cousins procedure : @xmath9 and @xmath10 . 
 a small but relatively energetic sample of partially contained @xmath0  events has also been isolated . 
 their distribution in log@xmath11 relative to null oscillation monte carlo is compatible with @xmath0  to @xmath3  oscillation scenarios within the parameters region allowed by our contained events .