  This paper describes the several phases which led, from the conceptual
design, prototyping, construction and tests with beam, to the installation and
operation of the BRAN (Beam RAte of Neutrals) relative luminosity monitors for
the LHC. The detectors have been operating since 2009 to contribute, optimize
and maintain the accelerator performance in the two high luminosity interaction
regions (IR), the IR1 (ATLAS) and the IR5 (CMS). The devices are gas ionization
chambers installed inside a neutral particle absorber 140 m away from the
Interaction Points in IR1 and IR5 and monitor the energy deposited by
electromagnetic showers produced by high-energy neutral particles from the
collisions. The detectors have the capability to resolve the bunch-by-bunch
luminosity at the 40 MHz bunch rate, as well as to survive the extreme level of
radiation during the nominal LHC operation. The devices have operated since the
early commissioning phase of the accelerator over a broad range of luminosities
reaching 1.4*10^34 cm-2 s-1 with a peak pileup of 45 events per bunch crossing.
Even though the nominal design luminosity of the LHC has been exceeded, the
BRAN is operating well. After describing the multiple applications that the
BRAN can be used to monitor the luminosity of the accelerator, we discuss the
technical choices that led to its construction and the different tests
performed prior to the installation in two IRs of the LHC. Performance
simulations are presented together with operational results obtained during p-p
operations, including runs at 40 MHz bunch rate, Pb-Pb operations and p-Pb
operations.

Accelerator Physics;High Energy Physics - Experiment;Instrumentation and Detectors