  The high design luminosity of the SuperKEKB electron-positron collider is
expected to result in challenging levels of beam-induced backgrounds in the
interaction region. Properly simulating and mitigating these backgrounds is
critical to the success of the Belle~II experiment. We report on measurements
performed with a suite of dedicated beam background detectors, collectively
known as BEAST II, during the so-called Phase 1 commissioning run of SuperKEKB
in 2016, which involved operation of both the high energy ring (HER) of 7 GeV
electrons as well as the low energy ring (LER) of 4 GeV positrons. We describe
the BEAST II detector systems, the simulation of beam backgrounds, and the
measurements performed. The measurements include standard ones of dose rates
versus accelerator conditions, and more novel investigations, such as
bunch-by-bunch measurements of injection backgrounds and measurements sensitive
to the energy spectrum and angular distribution of fast neutrons. We observe
beam-gas, Touschek, beam-dust, and injection backgrounds. We do not observe
significant synchrotron radiation, as expected. Measured LER beam-gas
backgrounds and Touschek backgrounds in both rings are slightly elevated, on
average three times larger than the levels predicted by simulation. HER
beam-gas backgrounds are on on average two orders of magnitude larger than
predicted. Systematic uncertainties and channel-to-channel variations are
large, so that these excesses constitute only 1-2 sigma level effects. Neutron
background rates are higher than predicted and should be studied further. We
will measure the remaining beam background processes, due to colliding beams,
in the imminent commissioning Phase 2. These backgrounds are expected to be the
most critical for Belle II, to the point of necessitating replacement of
detector components during the Phase 3 (full-luminosity) operation of SuperKEB.
