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the photoproduction of a pseudo scalar meson such as the kaon can be described by four complex amplitudes which result in sixteen real observables @xcite .
measurement of at least eight well chosen observables gives a `` complete measurement '' which is sufficient to fully constrain these photoproduction amplitudes .
currently this is an important goal of the leading photon beam facilities and measurements with combinations of polarised beam , polarised targets and @xmath1 recoil polarisation are being carried out , with the main physics motivation to better establish the nucleon excitation spectrum . as well as
resonance information , accurate data in the threshold region is needed to challenge recent theoretical models based on effective field theories .
there is presently no detailed measurement of the @xmath2 in the threshold region . at higher energies new quality data
is available from jlab @xcite and elsa@xcite .
however , significant discrepancies exist between the cross sections from the two previous measurements for photon energy ranges accessible at mami @xcite .
the jlab measurement also extracted the total polarisation transferred to the @xmath1 after photoproduction with a circular polarised photon beam .
this observable was close to one for all measured kinematics and for photon beam energies 1.032 to 2.741  gev which has led to speculation about the reaction mechanism @xcite .
we present preliminary spectra from the @xmath2 reaction with a circular polarised tagged photon beam in the energy range 0.9 - 1.4  gev using the crystal ball at mami - c .
the mainz microtron ( mami - c ) is an electron accelerator facility in mainz , germany , capable of accelerating electrons up to 1.5  gev .
the electrons produce circularly or linearly polarised photons via bremsstrahlung in a solid radiator .
the photon energy is tagged by momentum analysis of the recoiling bremsstrahlung electrons in the glasgow tagger @xcite .
the crystal ball @xcite is a calorimeter with 672 optically isolated nai crystals .
the output of the photomultiplier for each crystal is digitised using multi - hit tdc and adc modules .
surrounding the liquid hydrogen target at the centre of the crystal ball is the particle identification detector ( pid ) .
the pid comprises 24 plastic scintillator detectors parallel to the beam and read out by photomultipliers at the upstream end .
@xmath3 analysis provides a means of proton , charged pion and electron identification .
the crystal ball is an excellent neutral meson detector via the detection of the decay photons .
taps @xcite is a 350 element baf@xmath4 spectrometer which covers the forward angle range . the crystal ball and taps
cover approximately 93% of 4@xmath5 steradian .
the combined setup is shown in fig .
[ fig : cbpic ] .
it is not possible to identify @xmath0 using the pid detector alone as the particles have mass and charge too similar to the proton to be effectively separated using @xmath3 techniques and the subsequent weak decay of the particles in the detectors spoils the unique energy signal . in previous measurements
large magnetic fields and tracking detectors were employed to circumvent this problem . for the case of the crystal ball
this was impractical due to the geometrical constraints of the detector .
a new technique was therefore developed by our group to realise these important strangeness measurements .
the basic principle of the technique is illustrated in fig .
[ fig : cbpic ] .
the @xmath0 meson enters the crystal ball where it is stopped .
the crystals involved in this initial interaction are identified as those having a timing within 3  ns of the reaction time .
the meson subsequently can decay weakly via : @xmath6 or @xmath7 processes with branching ratios 64% and 36% respectively and a half life of 12  ns .
the characteristic energy deposition arising from these processes can clearly be identified in the energy spectra ( fig .
[ fig : strangemedley](a ) ) .
the timing of the delayed weak decay can be resolved using the 2  ns timing resolution of the crystal ball detector elements .
in fact the half life of the kaon can be extracted from observing the count rate as a function of time .
the result compares favourably with the accepted value ( fig .
[ fig : strangemedley](b ) ) .
the crystals in the secondary cluster are identified with a timing gate of between 10 - 50  ns later than the kaon entry time .
a geant4 simulation is used to optimise upper and lower limits on parameters such as the number of crystals in the cluster from the stopped @xmath0 and the @xmath0 decay , the distance to the furthest crystal and the total energy deposition . a subsequent @xmath8 analysis using the pid
provides a final constraint to suppress background from charged particles .
[ fig : strangemedley](c ) shows the reconstructed missing mass of the system recoiling from the @xmath0 which has clear peaks at the masses of the @xmath1 and @xmath9 , and fig .
[ fig : strangemedley](d ) is the energy of the photon beam for events in which a @xmath0 decay cluster occurred .
the number of events below the threshold for strangeness production at 911  mev averages very close to zero . the geant4 simulations of the detection process indicate the efficiency for the @xmath0 decay cluster method is approximately 8% .
the @xmath1 decays to either @xmath10 or @xmath11 with branching ratios approximately 64  % and 36  % respectively .
these are weak decays and parity violating . as a result
, the @xmath1 polarisation can be measured via the distribution of its decay particles .
[ fig : asymm ] is the constructed beam helicity asymmetry , @xmath12 , as a function of the polar angle of the decay neutron from the @xmath1 .
the plot shows clearly the expected modulation .
these spectra will be analysed to extract the polarisation transfer observables cx and cz .
we have developed a new techique for tagging strangeness which enables new accurate measurements of strangeness photoproduction using the crystal ball at mami - c .
further production data will be obtained in the future to improve the statistical precision of the data set
. the mami measurement will provide accurate data in the threshold region and the new measurements at higher egamma will help to resolve present discrepancies between the world data .
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the photoproduction of @xmath0 mesons is an important challenge to recent qcd based chiral perturbation theories in the strange quark sector and is an important constraint on the nucleon excitation spectrum . 
 we present preliminary data from a new high precision measurement using the crystal ball detector . the measurement pioneers a new technique for tagging strangeness using detailed cluster analysis in segmented calorimeters which has potential wider application at present and future hadron physics facilities .