  The 340-year old supernova remnant Cassiopeia A at 3.4 kpc distance is the
best-studied young core-collapse supernova remnant. Nucleosynthesis yields in
radioactive isotopes have been studied with different methods, in particular
for production and ejection of $^{44}$Ti and $^{56}$Ni which originate from the
innermost regions of the supernova. $^{44}$Ti was first discovered in this
remnant, but is not seen consistently in other core-collapse sources. We
analyse the observations accumulated with the SPI spectrometer on INTEGRAL,
together with an improved instrumental background method, to achieve high
spectroscopic resolution which enables interpretation towards a velocity
constraint on $^{44}$Ti ejecta from the 1.157 MeV $\gamma$-ray line of the
$^{44}$Sc decay. We observe both the hard X-ray line at 78 keV and the
$\gamma$-ray line at 1157 keV from the $^{44}$Ti decay chain, at a combined
significance of 3.8 $\sigma$. Measured fluxes are
$(2.1\pm0.4)~10^{-5}~\mathrm{ph~cm^{-2}~s^{-1}}$ and
$(3.5\pm1.2)~10^{-5}~\mathrm{ph~cm^{-2}~s^{-1}}$, which corresponds to
$(1.5\pm0.4)~10^{-4}$ and $(2.4\pm0.9)~10^{-4}~\mathrm{M}_{\odot}$ of
$^{44}$Ti, respectively. The measured Doppler broadening of the lines implies
expansion velocities of $4300$ and $2200~\mathrm{km~s^{-1}}$, respectively.
Combining our results with previous studies, we determine a more precise
estimate of ejected $^{44}$Ti of $(1.37\pm0.19)~10^{-4}~\mathrm{M}_{\odot}$.
The measurements of both lines are consistent with previous studies. The flux
in the line originating from excited $^{44}$Ca is significantly higher than the
flux determined in the lines from $^{44}$Sc. Cosmic ray acceleration within the
supernova remnant may be responsible for an additional contribution to this
line from nuclear de-excitation following energetic particle collisions in the
remnant and swept-up material.
