Excellent results for a first test with a fluorescent screen!
This type of x-ray unit is NOT capable of causing an acute (in the sense of delivering a fatal dose in seconds) radiation hazard. Very lage systems, like those used to scan whole cargo containers, may be acutely dangerous.
Acutely fatal doses don't even register on a simple geiger counter (it will just display overload, far far before you are in acute danger, because the tube can only detect a few hundred, maybe thousand, breakdowns a second).
It's important to understand orders of magnitude and units when dealing with ionizing radiation. Typical counters give a number in micro or milisieverts per HOUR. Normal background radiation is in the order of 2 mSv/YEAR depending a bit where you live.
That geiger counter will probably be blaring alarms at 2 mSv/HOUR, and you should not remain in such an environment for an hour given the option. So what if you decided to put your hand in the beam at that radiation level? Actually nothing if you did it for a short picture as is commonly done with x-rays in hospitals. There are 3600 seconds in an hour, so at 2 mSv/HOUR your looking at under 1 uSV for a second.
This doesn't mean you should be careless with these devices, especially if you use them often. The increased risk of cancer and such is certainly something to consider, but acute radiation sickness by the ouput of a dental or medical x-ray device is simply not possible, the lack the output power to do so.
Radiologists wear shielded cloths and/or hide behind screens because they are exposed several times every day increasing their risk of long term problems, not because they have -any- chance of getting acute sickness from a days work.
