Here is the wiki for HEAVY DUTY batteries, about halfway down the page read the heading on Leakage...http://en.wikipedia.org/wiki/Zinc%E2%80%93carbon_battery
Here the wiki for Lithium batteries, note the heading on Applications http://en.wikipedia.org/wiki/Lithium_batteries
Lithium batteries are a good choice if you are very observant that the voltage is correct. They often come in higher voltages than 1.5, the normal AA voltage. Some devices handle over voltage but don't try it on your expensive detector. Just keep in mind that Lithium batteries can be explosive if short curcuited, Several loose batteries in your pocket can short out so use the original container to transport them and treat drained Lithiums with caution for safety.
I made the comment to make sure your batteries are Alkaline, That should say....Make sure you do not use HEAVY DUTY batteries. Not long ago the choices were either heavy duty or alkaline but now they have designed a few that are better than alkaline such as Lithium.
I think the only difference you will find is the extra lifespan, so check the voltage and they should be fine. Lithiums are known to drop off sharply at the end of their lifespan. I don't know what voltage your detector will show when this happens but experience will eventually tell you when to expect the battery life to quickly die off. Plus another thing about Lithiums, they hold higher voltage closer to the end of their lifespan and drop quickly at the end of it, so you may NEVER see 'low bat' before they die. If you have ever used a Lithium powered drill gun you will definitely see what I am talking about, they are fine one moment and dead the next(not saying they have short life, but just die fast at the end of it). I would like to hear your experience when they become low or die.
To be sure, Are these the batteries you are asking about? http://www.atbatt.com/product/6363.asp
If so, they look to be a good choice since they are the correct voltage.