Good, but a few improvements...
1. It's spelled 'shields'.
2. When you get the white laser-type gun, the game still says 'partical gun'.
3. It's spelled 'particle'.
4. I, personally, would have liked it more had you had a brief invulnerability after getting hit, as opposed to constantly taking damage when in contact with an explosion field or an enemy. However, once I figured out you were throwing mines out in that asteroid field, I appreciated the way you did it more.
5. Call me crazy, but I don't like getting a weapon powerup that takes me back to the default weapon. I know some games do this, but if I wanted a pea-shooter I wouldn't pick up anything. Now, if gathering more weapon-ups of the same type increased the shot spread, that would be a different story...
All in all, pretty good.