1. Conditions at a potential location of a beam dump experiment at LCLS-II
Clive Field, SLAC: 4/29/16

2. --- following up on discussions at the Jlab BDX workshop about what might be possible at SLAC ---
At LCLS II, 929 kHz of 0.1 nC pulses => ~1022 e- on dump in a few years
The location for a dark particle detector already exists in a tunnel downstream of the future high power dump.
If detector signal’s leading edge can be timed to 1 nsec => achieve x1000 extra discrimination against random background.
But what are the backgrounds downstream of an e- dump?
Consider using LCLS-I to take a look.
Behind the present LCLS main dump, in the X-ray beam characterization room, FEE
Behind the iron shielding of the future BSY dump (Tor’s talk yesterday)
Inaccessible with beam on. By luck there were just enough long cables in place to get quick first impressions :--

3. LCLS II FACET II Cu linac switchyard Muon wall B A (NLCTA) (ESTB) UND.
Dump &
Downstream
Enclosure
(FEE)
UND.

4. Use LCLS main dump? LCLS Dump e- Beam at -50 Downstream of LCLS dump:
X-ray
beam
LCLS
Dump e-
Beam
at -50
Downstream of LCLS dump:
~10 degrees above dump axis

5. Beam Transport Hall
West End
Downstream of Muon Wall

6. LCLS-II Performance Parameters
SC Linac Based FEL System
Energy 4 GeV
Repetition Rate 929 kHz max.
Charge per Bunch 0.1 nC
X-ray Energy eV
FEL average Power 20 W
TWO undulators 2 beams
NC Linac Based FEL System (never to BSY dump)
Energy 15 GeV
Repetition Rate 120 Hz
Charge per Bunch 0.25 nC
X-ray Energy 1,000-25,000 eV
FEL Photon Energy 2 keV
HPS at SLAC August 14, 2015

7. LCLS II beam switching HXR Muon stopper BSY dump vertical under SXR

9. 13 meters

10. Looking upstream

11. Just downstream of muon wall - -
Equipment Access
Support Assy Weight
H: 71.2” W: 43.8”
2,343 Lbs.
New beam line
Beamline already
exists
Space for a detector under new beam line

12. LCLS BTH looking upbeam to muon wall

13. First drift length looking downstream

14. Dark beam view downstream of BSY target

15. -- Radiation induced in beam lines downstream of muon wall --
260 R/wk (8 cm S)
110 R/wk (30 cm S)
4.7 R/week @
DM detector location:
c/m
30 Rad/wk: 1900 c/m
30 Rad/wk: 1500 c/m
120 Rad/wk:1300 c/m
9x105 neutrons/sqcm/week

16. Counter signal at beam time
Counter signal near beam time
– caused by accel. dark current
(almost) no counter signals between
beam pulses 8 msec apart

17. December
January with 0.5 inch Pb shield

20. LCLS BTH looking upbeam to muon wall

21. BTH counter sensitivity not well measured
1 MIP (45°) is 1.7 MeV deposit
No sign of single muon pulse height “peak” in lab before installation.
Possibly ~ 150 mV. Cosmic ray signals were in ~100 mV range, even reaching 1.5 V
In tunnel, cable length reduces 150 mV to ~100 mV
20 mV ≡ ~ 400 keV deposit???
Can distinguish 2.5 mV PMT signal on a single ‘scope trace

22. Backgrounds for high energy recoils are not prohibitive so far, but now need a big step up in statistics – with calibrated counters.
Shielding study is needed for MeV-level backgrounds.
Next task will be to get approval and support for an installation with “quieter” cables and the ability to test different types of counter and shielding (e.g. CsI with a BDX SiPM).
And DAQ that’s appropriate to the detectors.
All work needs to be reviewed by and coordinated with LCLS staff. If it can contribute to LCLS characterization, so much the better.
And remember that accesses to the area occur only every few weeks. . .

23. Questions ?

24. Caution --
Trigger rate 122 Hz (every 8200 microsec.)

25. Lead glass ---
“Typical lead crystal glass is: 25% oxygen, 30% silicon, 25% lead, 10% sodium or potassium, 10% calcium, 1% other components such as iron, tin and fluorine”
Cherenkov photon yield should be about 260 photons per track-cm
well above threshold (for e- > 120 keV). A minimum ionizing
track should deposit about 7 MeV per cm.

26. Cosmic ray attenuation in soil (PNNL-20401)

27. FEE: downstream of LCLS main dump
With 1.1x109 /pulse -> dump, GeV, 120 Hz
Dosimetry measured 5.7 m downstream from shield wall (1.2 meters Fe, 0.9 meters concrete):---
0 ± 0.17 Rad/week (0± Gy/week)
6.1±0.8 x104 neutrons/sqcm per week (assuming MeV) --- relatively low.

28. FEE continued: pulse by pulse. . .
With scintillation counter (1 litre plastic, acrylic light pipe):--
At high e-beam energy, there’s a strong signal at beam time from scattered synchrotron radiation entering the room via the X-ray beam pipe
At low energy, very little signal at beam time
Signal rate not at beam time is much lower than at surface, consistent with cosmic rays
(limited characterization – had to trigger on beam-time signal from counter)

33. Fee counter not well characterized
Count rate in lab before installation:
20-30 Hz above 5 mV
Broad peak about 60 mV?
Pulse height spectrum extends to ~ 200 mV
In tunnel before beam
Few Hz above 5 mV
Even flatter pulse height distribution. Possible broad peak ~ 40 mV?
1 MIP = 6.6 MeV deposit (at 45°)
=> 5 mV ~ 800 keV???