Supernovae play a key role in the dynamics, structure, and chemical evolution
of galaxies.  The massive stars that end their lives as supernovae live for
short enough times so that many are still associated with dusty star formation
regions when they explode, making them difficult to observe at visible
wavelengths.  In active star forming regions (galactic nuclei and starburst
regions), visible extinction is especially severe.  Thus, determining the
supernova rate in active star forming regions of galaxies, where the supernova
rate can be one or two orders of magnitude higher than the average, has proven
to be difficult.  From spectra we obtained of SN1987A, we know that supernovae
have emission from a set of 4 strong lines in the mid-ir.  These lines include
[NiII] 6.63 um, [ArII] 6.98 um, Pf alpha 7.45 um, and [CoII] 10.52 um.
The [NiII] emission line is a unique supernova signature, and in SN1987A it
was the strongest line in the infrared for a period of a year and a half after
the explosion.  Since dust extinction is much less in the mid-ir than at
visible wavelengths (eg. A(6.63)/A(V) = 0.025), the NiII line can be used as a
sensitive probe to detect recent supernovae.  In the first ISO call for
proposals, we were awarded time to use ISOCAM to observe a selection of
infrared luminous nuclei of starburst galaxies to determine the supernova rate
in galactic nuclei and starburst regions.  Of the two galaxies for which we
have fully analyzed data, one of them, NGC5055, shows excess NiII emission
centered on the bright starburst nucleus (see Fig. 2).  We propose here to
obtain a second set of images, using ISOCAM, of our galaxy sample to measure
the decay in the NiII line intensity we expect in NGC5055 and any others we
might observe in the first set of data, and to enable us to discover any
supernovae which might have become visible in the intervening time.  A NiII
line intensity which does not decay with time might indicate that an excess of
Ni has been injected into the starburst region in the past rather than arising
from a single recent supernova.  ISO is essential for these observations since
the key supernova indicator, the 6.63 um [NiII] line, is unobservable from
any other existing observatory.