Friday, November 29, 2002, 16:00
WLGA E26/E28
Prof. W.I. Furman, JINR Dubna (On behalf of the DIANNA collaboration)
Abstract:
Due to the fact that recently a situation with quantitative measure of
charge symmetry violation in strong nucleon-nucleon interaction became
uncertain an unambiguous determination of neutron-neutron scattering
length a_nn is now actual. Very attractive possibility to obtain a_nn
value from direct measurement of neutron-neutron scattering provides the
powerful pulsed reactor YAGUAR operating in All-Russian Research Institute
of Technical Physics, Snezhinsk, Russia. In central channel of this
reactor the instantaneous thermal neutron flux 2.5 x 10^18 n/cm^2 s could
be realized so it is possible to have ~10^7 events of neutron-neutron
collisions per one reactor burst. Due to rather short pulse duration (700
micro s) time-of-flight method allows one to select neutron-neutron
scattering events. Extensive and detailed modeling of the experiment
carried out during last two years shows that in realistic geometry
providing acceptable background conditions it is possible to register more
than 102 useful events per burst. So a reasonable amount of reactor bursts
(realized not more than two times per day) permits to achieve a
statistical accuracy near 2.5% for a_nn value. But main limitations for
real precision should go from systematical uncertainties. Some preliminary
experiments have been carried out to measure absolute value of thermal
neutron density and its spatial distribution inside of the central channel
of YAGUAR reactor. Good agreement with results of simulation was achieved
but it is necessary to improve an accuracy of such measurements to get
desirable precision in determination of a_nn scattering length.
An estimation of various sources of background shows that most dangerous and indefinite one is the background arising by epithermal and far tail of fast (delayed) neutrons. The special test experiments are planning for first half of next year to measure real background, to verify respective MC calculations and to fix finally the geometry of main experiment. Extensive MC simulations are carried out to take into account time dependence of neutron fields, to optimize the collimation system and relevant shielding. But crucial information on feasibility of main experiment has to be obtained from series of test measurements at real experimental conditions near reactor YAGUAR.