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Calibration Techniquesfor

Dark Matter Experiments

J.A. FORMAGGIOMIT

Challenges of Calibrating Dark Matter Experiments

Dark Matter experiments propose a serious challenge with regard to calibrations:

Energy scales are often very small, and so low energy sources are often needed.

Often one wishes to know the response of a detector “everywhere” in the fiducial volume. Point sources do not necessarily work to this effect. So, one needs a diffuse source.

Need sources that cannot contaminate the detector or which are short-lived such that contamination is not an issue.

Two Sources

We consider two sources of calibration.

A pulsed neutron source to determine neutron detection efficiency.

A gaseous 83mKr source to determine gamma/electron backgrounds.

Pulsed neutron source Krypton Source Pulsed neutron source

POSSIBLE NEUTRON GENERATORS

POSSIBLE XH+YH REACTIONS:

D+D REACTION:

D + T REACTION:

T + T REACTION

FLAT SPECTRUM UP TO ~13 MEV

2H +2 H !3 He + n (En = 2.45MeV)

2H +3 H !4 He + n (En = 14.1MeV)

PROTOTYPE

Compact pulsed neutrons sources based on cold or hot emission (as well as RF) have been successful.

Consider similar system for MiniClean detector.

W. BARLETTA

Pulsed RF DT source

Pulsed cold DT source

SCHLUMBERGER COMPACT NEUTRON SOURCE

1. Used in oil industry for oil/material exploration.

2. Compact, pulsed HV source.

3. Uses pulsed ion source against HV target

For 30kV on DD target, expect ~105 neutrons/s

ENERGY DISTRIBUTION

1. ENERGY DISTRIBUTION NOT ENTIRELY MONO-ENERGETIC (ENERGY/ANGLE CORRELATION)

2. FOR LOWER VOLTAGES, ENERGY SPREAD MUCH NARROWER AND CAN BE MODELED.

Deuteron Energy 0o 90o 180o

50 keV

100 keV

150 keV

200 keV

2.723 MeV 2.462 MeV 2.225 MeV

2.852 MeV 2.474 MeV 2.146 MeV

2.958 MeV 2.486 MeV 2.090 MeV

3.052 MeV 2.498 MeV 2.045 MeV

COMPACT NEUTRON SOURCE

1. Components:

1. Electron Filament (3 V, 3A) -> 9W

2. Cathode (3V, 3A) -> 9W

3. Deuterium Ion Source (200 V, 200 ma, pulsed) -> 40 W

4. Steady HV source (30kV to 100 kV, up to 500 uA) --> 15W

Pulsed Electron emitter and Ion source

Target beam stop (copper)

Schlumberger Compact Neutron SourceUS Patent 5,293,410

Sharp Rise

30-100 kilovolts

1.2" Dia x 6" long

No Delay

D2 coated Ti Target

D2 Source and Electron Emitter

Pulsed Electron emitter and Ion source

Target beam stop (copper)

Schlumberger Compact Neutron SourceUS Patent 5,293,410

Sharp Rise

30-100 kilovolts

1.2" Dia x 6" long

No Delay

D2 coated Ti Target

D2 Source and Electron Emitter

Pulsed Neutron Source Diagram

SCHLUMBERGER COMPACT NEUTRON SOURCE

Advantages:

1. Compact enough to be inserted within the fiducial volume of the detector for recoil detection.

2. Allows option of low energy (D-D), high energy (D-T), and flat spectrum (T-T) source.

Disadvantages:

1. Limited lifetime (~1000 hrs operations).

2. Heating / HV issues.

3. Specific source at hand non-commercial.

Source characterization continuing this summer

Advantages of Diffuse Source

Study energy scale everywhere in fiducial volume.

If diffuse source is completely mixed, true test of detector acceptance and fiducial volume (since signal has same dependence)

In most cases, only a small amount of radioactivity is necessary (exception with TPC, where gamma rejection is high).

Successfully used in SNO (24Na for neutrons and 222Rn spike for low energy gammas) to calibrate detector.

Advantages of 83mKr

METASTABLE FORM OF KRYPTON-83 IS IDEALLY SUITED FOR EXPERIMENTS OF THIS TYPE (INCLUDES ARGON, XENON, AND GASEOUS TPC’S).

LOW ENERGY MONO-ENERGETIC ELECTRONS/GAMMAS:

SHORT HALF-LIFE (1.83 HOURS)

NOBLE GAS -- CAN BE DIFFUSED INTO GASEOUS AND LIQUID MEDIA.

Ideal for dark matter experiments!

1.83h 83mKr 1/2-

154ns 83Kr 7/2+

stable 83Kr 9/2+

32.1

9.4

7/2+

9/2+

17.8

17824.35±0.75 eVconversion electron

9.4

K-ionatom

86d 83Rbε

Conversion electrons at 30 and 32 keV also exist.

Already in use...

THIS TECHNIQUE IS ALREADY BEING USED.

KATRIN’S MAIN CALIBRATION USES 83mKr’s 17.8 keV ELECTRON FOR PRECISE CALIBRATION OF ITS ELECTRON TRANSPORT AND RESPONSE.

PROVIDES MONO-ENERGETIC ELECTRONS DIFFUSE INTO THE DETECTOR. IDENTICAL TO BETA DECAY SIGNAL, EXCEPT FOR TEMPERATURE (NEEDS TO OPERATE AT 120 K INSTEAD OF 27K).

KATRIN BEAMLINE

Production Methods

Can make the krypton isotope via 2 methods. Each involves making 83Rb:

1. Via 81Br(α,2n)83Rb. One bombards dissolved bromine salt with high energy alphas (alternatively, on pure bromine). This produces Rb isotopes.

2. Via 83Kr(p,n)83Rb using hydrogen ions as the main beam.

81Br(α,2n)83Rb 83Kr(p,n)83Rb

AdvantagesUse of existing beam facilities; no means to produce 81Kr and 85Kr

Possible easy access via research hospitals; clean

extraction

Disadvantages Beamline non trivial; difficult chemistry Purity of Kr cell

Extraction Methods

Rubidium can be washed off the krypton cell with water and passed through a zeolite.

Zeolite will retain all the Rb salt with no release of the Rb itself. Gaseous Krypton will escape from the matrix if pumped out, but without extracting rubidium itself.

Necessary to wait for other Rb isotopes to decay, usually gone in ~14 days (Rb lifetime 83 days).

Injection into liquid Xenon (Dan McKinsey, Yale U.) already successful; being expanded into liquid Argon.

L. W. Kastens, hep-physics 0905-1766

Conclusions

Both pulsed neutron sources and short-lived isotopes prove to be effective means of calibration liquid and gaseous dark matter detectors.

Radioactive krypton allows for low energy electron calibration of dark matter detector without serious risk of contamination.

These techniques should be applicable to both standard and directional-based detectors.