Majorana Neutrinos in the KamLAND-Zen I'm reading the following paper concerning the KamLAND-Zen experiment "Search for Majorana Neutrinos near the Inverted Mass Hierarchy Region with KamLAND-Zen" 
In the second paragraph of the first page they make reference to "...The outer LS acts as an active shield..." 
What is this active shield and what does it do?
 A: There is no general significance to the phrase "active shield".
In this instance it means that that particular layer of the detector provides two services:


*

*It simply acts to absorb most of the radiation impinging on the detector from the outside. (Actually there are also a couple of layers of steel and an outer buffer—not mentioned in the paper but familiar to anyone who has been following the history of KamLAND—that is apparently not used in the analysis on the XEN variant.)

*It in the event of penetrating charged radiation (i.e. high energy muons which are essentially the only external radiation that can pass through the external both the outer buffer and the inner buffer) it register's a signal so that the analysis won't mistake energy depositions in the xenon-doped scintilator as coming from a neutrino event. This kind of behavior is often referred to as a "active veto" or just "veto".

Full disclosure: I was a member of KamLAND for a few years back in the reactor neutrino days.
