1. Signal and Background in LENS
LONU-LENS
Mini-Workshop on Low-Energy Solar Neutrinos & LENS Blacksburg, VA, October 14
Christian Grieb
Virginia Tech

2. 7Be, CNO & LENS-Cal signals not affected by Indium-Bgd!
LENS-Indium: Signal
CC -capture in 115In to excited isomeric level in 115Sn
Tag: Delayed emission of (e/)+ 
Threshold: 114 keV  pp-’s
115In abundance: ~ 96%
CC-capture: Faithful
reproduction of  spectrum
Background Challenge:
Indium-target is radioactive! (t = 6x1014 y)
115In β-spectrum overlaps pp- signal
Basic background discriminator:
Time/space coincidence tag
Tag energy: E-tag = Eβmax +116 keV
Requires good spatial & energy resolution
7Be, CNO & LENS-Cal signals
not affected by Indium-Bgd!

3. LENS Expected Result: Low Energy Solar -Spectrum
>98% Flux <2MeV
Signal (t = 4.76 µs)
LENS-Sol Signal =
SSM(low CNO) + LMA x
Detection Efficiency e
Rate: pp 40 pp ev. /y /t In
2000 pp ev./ 5y/10t In  ±2.5%
Design Specification: S/N ≥ 3
pp: e = 64%
7Be: e = 85%
pep: e = 90%
Access to pp  spectral shape for the first time

4. Indium --Background Structure – Space / Time coincidence
Signal
Signal Signature:
Prompt e- ( )
followed by
low energy (e-/) ( )
and
Compton-scattered  ( )
->time/space coincidence
-> tag fixed energy 613keV
->compton scattered shower
E() -114 keV
116 keV
497 keV
115In
115Sn
e/ 
=4.76s
115In
β0 + n (BS)
(Emax = 499 keV)
498 keV
*Cattadori et al: 2003
β1 (Emax< 2 keV)
(b = 1.2x10-6)*
115Sn 
Background:
Random time and space coincidence
between two -decays ( );
Extended shower ( ) can be created by:
a) 498 keV  from decay to excited state;
b) Bremsstrahlungs -rays created by ;
c) Random coincidence (~10 ns) of more -decays;
Or any combination of a), b) and c).
Background

5. Indium Radioactivity Background
Background categories
n 115In –decays in (quasi) prompt coincidence produce a tag:
Basic tag candidate: Shower near vertex (Nhit ≥ 3) - chance coincident with 115In β in vertex
Type A: A1 = β + BS  (Etot = 498 keV) (x1)
A2 =  (498 keV) (x1)
Type B: 2 β-decays (x10-8)
Type C: 3 β-decays (x10-16)
Type D: 4 β-decays (x10-24)
Strong suppression via energy
Suppression via tag topology
Example Type C:
3 βs in quasi-prompt (~10ns)
coincidence

6. Indium Background Rates
pp Signal
/y /t In
Bgd tot
Bgd A1
Bgd A2
Bgd B
Bgd C
RAW
62.5
79 x 1011
Valid tag (Energy, Branching, Shower) in Space/Time delayed coinc. with prompt event in vertex 55
2.76 x 105
8.3 x 104
2.8 x 103
1.9 x 105
43.9
+ ≥3 Hits in tag shower 46
2.96 x 104
2.6 x 104
2.5 x 103
1.4 x 103
43.7
+Tag Energy = 613 keV 44
297
0.38
4.5
284
8.1
+Shower Radius
42.7
104 98
0.74
+Hit Separation
39.4
11.5 ±0.5
4.1
7.1
0.007
Valid Tag:
Time and Space coincidence:
Two events within 10s inside the same volume (cubic cell 7.5cm)
Spatial Resolution: assume cubes 7.5 x 7.5 x 7.5 cm3
Energy Resolution: E/E 4.3% at 613 keV (900 pe / MeV)
Only for A1:  energy > 450keV and has to create at least one Bremsstrahlungs  > 40keV
Separation of A1 Background and Signal Tag:
E=115 keV or E/E > 5

7. Indium Beta Spectrum & Bremsstrahlung
115In --spectrum
- spectrum for
E+E  450 keV
after production of
at least 1 BS 
with E  40keV
8.610-3
9.010-3
E [MeV]
E [keV]
E [keV] N
BS  spectrum for E+E  450 keV
after production of at least
1 BS  with E  40keV
Type A1 Background:
Consider only events where E+E  450 keV and at least one BS  with E  40keV is produced
 reduce computing time by
9.010-3 x 8.610-3 = 7.7 10-5

8. Hit Multiplicity (“Nhit”)
Example: Event with
Nhit = 3 n
Nhit distribution for
-event tags
Demand ≥3 Hits
in tag shower
pp Signal
/y /t In
Bgd tot
Bgd A1
Bgd A2
Bgd B
Bgd C
RAW
62.5
79 x 1011
Valid tag 55
2.76 x 105
8.3 x 104
2.8 x 103
1.9 x 105
43.9
+ ≥3 Hits in tag
49.5
6.23 x 104
5.81 x 104
2.76 x 103
1.4 x 103
43.8
Nhit n
Nhit distribution for
In-Background
Classify all events according to Nhit
Optimize Cut Parameters individually for each class
Nhit

9. Tag Energy 115In Total Energy deposit in Tag A1 Bkgd
β0 + n (BS)
(Emax = 499 keV)
498 keV
*Cattadori et al: 2003
β1 (Emax< 2 keV)
(b = 1.2x10-6)*
115Sn 
Total Energy deposit in Tag
A1 Bkgd
A2 Bkgd
-Events (pp)
N year-1 t In-1 keV-1
B Bkgd
C Bkgd
D Bkgd
Powerful energy
Separation for A1 & A2
Background
Not so powerful for B,C,D
Mention that pp signal only applies to the rate, of course other neutrinos will produce exactly the same tag
Etag [keV]

10. Rates after Tag Energy Cut
pp Signal
/y /t In
Bgd tot
Bgd A1
Bgd A2
Bgd B
Bgd C
RAW
62.5
79 x 1011
Valid tag (Energy, Branching, Shower) in Space/Time delayed coinc. with prompt event in vertex 55
2.76 x 105
8.3 x 104
2.8 x 103
1.9 x 105
43.9
+ ≥3 Hits in tag shower
49.5
6.23 x 104
5.81 x 104
2.76 x 103
1.4 x 103
43.8
+Tag Energy = 613 keV
44.4
458
0.48
5.2
445
8.0
Type A1 / A2 Background: ok
Type B / C: still problematic

11. Shower Radius
Shower radius R for  event tags is defined by propagation of 497 keV  in the scintillator
Shower radius for A2 Bkgd is naturally identical (498 keV )
For B, C Bkgd random coincidence rate scales with allowed shower volume N
Tag Shower Radius distribution
B Bkgd
-Events (pp)
A2 Bkgd
C Bkgd
pp Signal
/y /t In
Bgd tot
Bgd A1
Bgd A2
Bgd B
Bgd C
RAW
62.5
79 x 1011
Valid tag 55
2.76 x 105
8.3 x 104
2.8 x 103
1.9 x 105
43.9
+ NHit ≥3
49.5
6.23 x 104
5.81 x 104
2.76 x 103
1.4 x 103
43.8
+Tag Energy = 613 keV
44.4
458
0.48
5.2
445
8.0
+Shower Radius
270
5.1
265
0.73
R [mm]
Mention: erratic distribution due to digital spatial resolution, only discrete values for distances available R

12. Hit Separation Identification of remaining Type B Background x N
E [keV] N
BS  spectrum for E+E  450 keV
after production of at least
1 BS  with E  40keV
Most likely case: one of the two s produces a low energy BS  ray.
The second  decay (2) occurs randomly and is separated from the shower produced by 1.
Type B scales with the allowed volume, i.e. x3 N
Hit Separation Distribution
1:Emission of low E BS  ray
2: ”lone ”,
randomly
Coincident
within <10ns x x

13. Signal and In-Background Rates
pp Signal
/y /t In
Bgd tot
Bgd A1
Bgd A2
Bgd B
Bgd C
RAW
62.5
79 x 1011
Valid tag (Energy, Branching, Shower) in Space/Time delayed coinc. with prompt event in vertex 55
2.76 x 105
8.3 x 104
2.8 x 103
1.9 x 105
43.9
+ ≥3 Hits in tag shower
49.5
6.23 x 104
5.81 x 104
2.76 x 103
1.4 x 103
43.7
+Tag Energy = 613 keV
44.4
458
0.48
5.2
445
8.0
+Shower Radius
270
5.1
264
0.73
+Hit Separation
40.2
13.3 ±0.6
4.7
8.1
0.004
Signal / Background ~3 with pp- event detection efficiency 64%
Remember: only pp- events affected by Indium Background, 7Be, pep and CNO Background-free

14. pp-Signal and In-Background revisited
Shower Radius R
Total Energy deposit
in Tag
Black: pp  events
Blue: A1 Bgd
Green: A2 Bkgd
Red: C Bkgd
N year-1 t In-1 2keV-1
R [mm] N
Hit Separation x
E [keV]
x [mm]

15. LENS Design Figures of Merit
Cell Size [mm]
Cube size [M]
pe/
MeV
Det. Eff [%] Nu
/t In/y
Bgd
S/N
M (In)
[tons]
M (InLS) tons
PMTs 75 5
900 64 40 13 3 10
125
13300*
(3”) ~6
950 26 9
2.9
15.3
190
6250*
(5”)
*Pmt’s on three sides only
LENS is a feasible detector,
125t of liquid scintillator and ~13300 photomultiplier channels
for ~2000 pp- events in 5 years with full spectroscopic information plus 7Be, pep and CNO

16. Additional Slides