Pu-241
Pu-241 is isotope of Plutonium.
mass: 241.057 amu
Production
U-238 + n $ \rightarrow$ Np-239 + $\beta^-$
Np-239 $ \xrightarrow[~]{\beta^-}$ Pu-239
Pu-239 + n $ \rightarrow$ Pu-240
Pu-240 + n $ \rightarrow$ Pu-241
Most or nuclear reactors have significant concentration of U-238 in the active rods; so, production of plutonium happens automatically, whether it is wanted of not.
At accidents with nuclear plants, isotopes of Plutonium appear as one of main contaminants. Then, it decays to Americium, that becomes the main contaminant at the scale larger than a century.
Using
$ \rm ^{241}_{~94}Pu \xrightarrow[15.4~y]{\beta^-,~ 21\,KeV} {} ^{241}_{~95}Am \xrightarrow[432.7\,y]{\alpha,\, 4.959\,MeV} {}^{237}_{~96}Np \xrightarrow[2.4 \times 10^6\,y]{\alpha,~4.958 \,MeV} \mathrm{^{233}_{91}Pa} ~.\,.\,. $
References
https://en.wikipedia.org/wiki/Plutonium-241