The TGRS experiment on board the Wind spacecraft has many advantages as a sky monitor — broad field of view ( \sim 2 \pi centered on the south ecliptic pole ) , long life ( 1994–present ) , and stable low background and continuous coverage due to Wind ’ s high altitude high eccentricity orbit .
The Ge detector has sufficient energy resolution ( 3–4 keV at 511 keV ) to resolve a cosmic positron annihilation line from the strong background annihilation line from \beta -decays induced by cosmic ray impacts on the instrument , if the cosmic line is Doppler-shifted by this amount .
Such lines ( blueshifted ) are predicted from nucleosynthesis in classical novae .
We have searched the entire TGRS database for 1995–1997 for this line , with negative results .
In principle such a search could yield an unbiased upper limit on the highly-uncertain Galactic nova rate .
We carefully examined the times around the known nova events during this period , also with negative results .
The upper limit on the nova line flux in a 6-hr interval is typically < 3.8 \times 10 ^ { -3 } photon cm ^ { -2 } s ^ { -1 } ( 4.6 \sigma ) .
We performed the same analysis for times around the outburst of Nova Vel 1999 , obtaining a worse limit due to recent degradation of the detector response caused by cosmic ray induced damage .