  The Gamma-Ray Observation of Winter THunderclouds (GROWTH) experiment,
consisting of two radiation-detection subsystems, has been operating since 2006
on the premises of Kashiwazaki-Kariwa nuclear power plant located at the
coastal area of Japan Sea. By 2010 February, GROWTH detected 7 long-duration
$\gamma$-rays emissions associated with winter thunderstorms. Of them, two
events, obtained on 2007 December 13 and 2008 December 25, are reported.On both
occasions, all inorganic scintillators (NaI, CsI, and BGO) of the two
subsystems detected significant gamma-ray signals lasting for >1 minute.
Neither of these two events were associated with any lightning. In both cases,
the gamma-ray energy spectra extend to 10 MeV, suggesting that the detected
gamma-rays are produced by relativistic electrons via bremsstrahlung. Assuming
that the initial photon spectrum at the source is expressed by a power-law
function,the observed photons can be interpreted as being radiated from a
source located at a distance of 290-560 m for the 2007 event and 110-690 m for
the 2008 one, both at 90% confidence level.Employing these photon spectra, the
number of relativistic electrons is estimated as 10^9 - 10^{11}. The estimation
generally agrees with those calculated based on the relativistic runaway
electron avalanche model. A GROWTH photon spectrum, summed over 3 individual
events including the present two events and another reported previously, has
similar features including a cut-off energy, to an averaged spectrum of
terrestrial gamma-ray flashes.
