You probably won't find a public spec on near-field EMI as this is a complex geometrical and complex math curl product of emissions * susceptibility threshold * the mutual coupling impedance loss between.
You are looking for near field EMC specs and these are too variant with sub-system design geometry and shielding. System level tests usually done at 10~30 m outside and inside the box you can sniff with near-field loop probes from 10~30 mm for current noise spectrum using various size loops that trade-off sensitivity with bandwidth.
I have designed many complex systems with EMC issues and resolved all of them with instruments such as solenoid-pneumatic robotics with Eddy Current guided at 100k/200kHz as well as designed and performed Corporate System level susceptibility tests for conducted ESD and power line noise, Magnetic current levels from 10Hz to 30MHz , E field interference levels with AM over RF up to microwave with high E levels, ESD impulse conducted to frame and radiated from a plane wave conductor sheet.
These were done on a variety of products such as magnetic disk drives, DS1 transceivers, and thresholds were established by changes in analog window timing margin from injected jitter ( starting from >30% minimum margin) which translates to digital bit error rate (BER) <<1e-12 or very high SNR's.
Initially in the 70's and early 80's I used real-world noise generators like brushed DC motors and ESD discharge generators and then refined the process with dV/dt and dI/dt controlled spectral density from some fundamental swept frequency clock rate into the late 80's.
What I discovered was that the effective shielding of Disk drives was great but the ingress on long cables was poor which became most apparent with AM modulated 1GHz being rectified by ESD protection diodes and other non-linear impedances. Meaning feedthru caps were a necessary part of the design.
But in your case it appears to be surface currents on the photo-tube and interface cable power noise with sensitive circuit impedance or perhaps conductive supply ripple vs frequency which then is limited by decoupling cap ESR.
We also discovered the ground inductance and resistance of earthed coaxial CATV cable greatly affected the ingress from motive power trains within a block of the parallel coax and this affected RF video signal integrity on analog TV.
So my advice is perform margin tests to swept frequency E fields and H fields with linear and loop antenna using modulated carriers to simulate CMOS transition speeds or use square waves in to capacitive loaded loops to define your modules sensitivity on cable interfaces and shielded modules. Then you have measureable criteria to determine allowable limits at a specified short range perhaps using 1cm gap and a 10cm wire or a 10cm Loop at a 5cm gap.
Also recognize that slots or gaps are just as effective as antenna between module shields as linear wires but ultimately your geometry and your sensitivity to ingress ultimately affects your acceptance and failure limits. These ought to be measureable in some analog form such as % margin, SNR , voltage ripple, or spectral density below the wanted signal at some defined power level, area and gap.
You can get fibre optic E-field meters normally used inside Faraday Cages for measuring such sub-system egress or applied external ingress but remember that Helmholz Resonators from standing waves inside a shielded enclosure will create nulls on an E-Field detector but limiting these anomalies, you can compare applied noise levels with a simple short whip antenna or wire for calibrating your susceptibility tests.
I recall 30 yrs ago we used more than 10V/m and more than 20A/m in AC to RF susceptibility tests, but don't hold me to that value... It's been a while.
When we didn't have specs, we took responsibility for assuming the worst and designed then verified the results.
( The only time I recall failing was not being able to live test a telemetry SCADA system which I designed in the 70's with a 1 mile RS485 cable and launch control's AM RF radio caused bit errors when used causing unexpected results, long after I left the company for a 25% pay raise.)
Regarding Line filter noise injected into the chassis gnd. Proper isolation of this sub 10kHz noise can be treated with suitable RF isolation circuits to your internal shields such that they become 0V RF gnds in a star configuration or controlling the discharge currents impedances so they exit the box rather than radiate inside. External cable ingress is often your worst nightmare from ESD and power line floating switchers with coupling capacitance in the switcher transformer. But keep in mind Hipot tests with grounded secondary is not done by an OEM supply and this is more stressful on primary insulation for the same reasons ( SMPS XFMR Coupling C)
This probe consists of a small coil which is shielded
against electrical fields and isolated on the outside for
safety reasons. Two examples are shown below
1D and 3D EMI sniffer probes.
The first probe has a linear coil, causing it to be most
sensitive across the length of the probe. The second
probe has three coils, which are perpendicular to each
other and electrically connected in series. Its sensitivity
to H-fields is independent of the field orientation. It
depends on the particular case to determine which probe
is best, but in general, the 3D probe is used to “scan” the
application for locations with high field intensity, and
the 1D probe is used to get a more precise picture of the
nature of the problem.
If you are really pushed for time and desperate for a spec, use the Automotive sub-system specs for EMI and susceptibility in your location or use USA MIL-standards which with suppliers for references were the earliest source standards for EMC IMHO :)