June 27, 2018 1

DAMIC-M Cryostat Conceptual Design, ULB Materials Production and Fabrication,Recent ULB Cryostat Examples

TODD HOSSBACHwith contributions from my PNNL colleagues

DAMIC-M Collaboration Meeting – June 2018 – Paris

PNNL-SA-135464

Outline

Review of the conceptual DAMIC-M cryostat design

Copper electroforming capabilities

Clean fabrication and assembly

Examples of recent ultra-low-background cryostats produced at PNNL

Design, fabrication, assembly

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 2

Conceptual Design Principles

Guiding principles

Begin with what we know works

Minimize complexity

Focus on background control

DAMIC-M design particulars

Use of electroformed copper important in achieving background goal

To achieve our timeline and stay within budget (background and €/$), high-quality OFHC copper should be used in non-

critical components of cryostat

Flex cable management and associated background mitigation

Vacuum feedthroughs

Stable experimental operation

Good thermal contact between CCDs and heat conductors

Minimize IR load on sensors

Re-condense N2 boil-off gas from Dewar

Critical for us to define the CCD packaging and detector module as soon as possible.

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 3

DAMIC-M Cryostat Design Concept –From ERC/NSF Proposal

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 4

DAMIC-M Cryostat Design Concept –Some recent updates

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 5

• 33 x 33 x 38cm inner volume

• Cu, within shield, is OFHC

• Alternative location for vacuum feedthrough

• Sterling/PT cooler for N2

reliquification

Shielding design based on C4 Dark Matter Experiment• Graded Pb purity• Optional inner Cu

liner for support• Neutron moderator /

borated poly

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 6

Electroforming

AC plating process

Copper growth ~0.0015” (38

microns) per day

Have grown large pieces >1”

(25mm) thick

Begin with highest purity OFHC

starting stock

Uses Optima-grade acid

Typically growth on cylindrical

and hexagonal mandrels.

Routine assay of electrolyte

Electrochemical Purification of Copper

Originally developed in the 1980’s as a method to reduce trace radioactive impurities in HPGe copper cryostats

Significant improvements to electroforming methods in early 2000’s

Reduced U/Th levels, better materials properties, reproducible growth, new cleaning and surface passivation

Most radiopure copper available <0.1 µBq U or Th/kg Cu

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 7

H2SO4

Man

dre

l(c

ath

od

e)

OFH

C C

u (a

no

de)

Power SupplyPUMP

Filt

er

Eric HoppeElectroforming Lead

Copper Electroforming at PNNL

Electroforming occurs at 35 mwe in dedicated Class 1000 cleanroom – 10x reduction in production

of cosmogenics

14 Electroforming Baths

8 small baths – mandrels up to 6” (15cm) diameter and 16” (40cm) long

6 large baths – mandrels up to 13” (33cm) diameter and 24” (60cm) long

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 8

Copper Electroforming at PNNL

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 9

https://tour.pnnl.gov/shallow-lab.html

Physical Properties of Electroformed Copper

10

Electroformed copper shows consistency in mechanical response from tensile and hardness testing on different regions across the material profileCurrently developing radiopureCu/Cr alloy with mechanical properties of mild steel

Electroformed copper is evaluated for grain size, hardness, and tensile strength(most commercial OFHC copper is <10 ksi)

0

50

100

150

200

250

0 0.08 0.16 0.24 0.32 0.4 0.48

Stress vs. Strain

First Thick Majorana Plate

T1T2T3M1M2M3B1B3

S1 Strain mm/mm

Yield Strength: 13.78 ± 0.58 ksiUltimate Tensile Strength: 31.2 ± 0.73 ksiYoung’s Modulus: 9.34E3 ± 1.74E3 ksi

• Significant Strain hardening • Significant Ductility

Growth Direction

µm

HV

Hardness measured across a surface Hardness contour map across a surface

Photo of microindent

Consistent tensile strength from multiple samples

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 11

Fabrication

Clean Fabrication of Copper and Polymers

Three specially trained professional machinists

Follow defined handling protocols – gloved hands, face masks, etc.

Two dedicated clean (low-background) machine shops

CNC mill, knee mill, lathe, EDM (electrical discharge machining) hole-drilling/die-sinking

One general purpose machine shop

CNC mills, knee mills, lathes, wire EDM, band saw, electron-beam welder, etc.

Aggressive cleaning of existing tools, tooling, fixtures, etc. prior to use in low-background applications

Fixtures made of compatible materials (e.g. avoid Al if cutting Cu)

Vacuum fixtures used to produce DAMIC parts

Alternative cutting fluids and coolants

Propylene glycol, cryo-machining (liquid nitrogen), vortex coolers (-46C)

Proper storage between fabrication steps

Post-fabrication cleaning – acidified peroxide solution (removes 1 micron/min)

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 12

Clean Fabrication of Copper and Polymers

Machine tools located in typical laboratory environment – not a cleanroom

Access to tools strictly controlled

Only low-background raw materials permitted in shop

Final parts marked with a unique identifier using laser engraving system

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 13

Clean Fabrication of Copper and Polymers

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 14

EDM fine-hole drilling and die sinking• New capability at PNNL• EDM process can be extremely clean • Only 75 units produced by

manufacturer• Capable of drilling extremely fine

holes in all metals• Die-sinking enables production of

parts/finishes not possible with conventional machining methods

• Pipes for drilling or electrodes for die-sinking can be made from high-purity copper

Fabrication of OFHC Copper Cryostat Parts and DAMIC Electroformed Copper CCD Modules

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 15

1% H2SO4 (optima)3% H2O2 (optima)

CLEANING!

Clean Joining of Detector Elements –Electron-Beam Welding

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 16

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 17

Examples of Recent ULB Germanium Spectrometer Production

RECENT PRODUCTION OF ULB HPGe SPECTROMETERS

In 2012 and 2017, we produced two ultra-low-background HPGe

spectrometers

C4 dark matter experiment, ULB gamma assay

First cryostats produced at PNNL to involve significant mechanical,

thermal, and electrical modeling prior to beginning production.

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 18

radius

7.830

6.264

4.698

3.132

1.566

0.000

3500

2800

2100

1400

700

0

Voltage U (V)

Strength E (105 V/m)

cry

stal

cyli

ndri

cal

axis

COMSOL Model with Resistive Joints

Horizontal Cold Finger (HCF)

Vertical Cold Finger (VCF)

Cold Plate (CP)

90.5

90

89.5

89

Kelvin

91

91.5

92

(not visible)

Model components

and sensor locations

C4 Cryostat - 2012

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 19

CRYOSTAT

DIPSTICK

PREAMPLIFIER

CROSS ARM ENDCAP

ENTRANCEWINDOW

Anatomy of a Germanium Spectrometer

C4 Cryostat – 2012Step 0 – Perform calculations / modeling

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 20

COMSOL FEM Model

COMSOL FEM modeling was used to

determine how changes to the

thermal path could lower the

operating temperature of the crystal in

future cryostats.

1-D Model – Electrical Analogy

1- 0 1 2 3 4 5 6 7 8 9 10 11 12 13 1470

72

74

76

78

80

82

84

86

88

90

92

94

96

98

100

0

2

4

6

8

10

Nodal Temperature Distribution

Node Number

Tem

pera

ture

(K

)

Tem

pera

ture

Dif

fere

nti

al

(K)

A one dimensional model was used to

estimate the various thermal loads and

final crystal temperature.

C4 Cryostat – 2012Step 1 – Electroformed copper growth, fabrication of components

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 21

C4 Cryostat – 2012Step 2 – Test assembly of thermal path and vacuum jacket followed by leak testing

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 22

C4 Cryostat – 2012Step 3 – Electrical wiring

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 23

Low Voltage High Voltage

Stainless fasteners used only in test assemblies. Replaced with copper in final assembly.

Brass blank used as surrogate for Ge crystal in test assembly.

C4 Cryostat – 2012Step 4 – Thermal testing

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 24

C4 Cryostat – 2012Step 4 – Thermal testing

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 25

Final crystal temperature of 91.5K reached in 8 days.

Crystal reaches 100K in ~3.5 days

C4 Cryostat – 2012Step 5 – Germanium integration

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 26

ULB Gamma Spectrometer– 2017

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 27

The image shown at the right is a

thermal profile of the cryostat that

indicates a ~6K temperature variation

along the length of the cryostat.

Modeling Case:

1-1/8” thermal conductor diameters

Modeling Outcomes:

Cold-plate temperature: 84K

IR loading: 1.6W

Total LN load: 3.7W

LN Consumption: 2L/24hr

ULB Gamma Spectrometer– 2017Improving thermal performance

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 28

Cryostat thermal performance had to be improved due to the higher IR load (larger HPGe crystal) and longer horizontal cross arm

ULB Gamma Spectrometer– 2017Critical design feature to eliminate thermal resistance of joints

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 29

No measurable thermal resistance with tapered joints

ULB Gamma Spectrometer– 2017Blending electroformed and OFHC copper

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 30

By using OFHC copper where possible, cryostat production time

can be significantly reduced

OFH

C C

u

E-Cu

ULB Gamma Spectrometer– 2017Electrical wiring

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 31

ULB Gamma Spectrometer– 2017Lower background electronics

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 32

PNNL developed low-background front-end electronics for use in HPGe spectrometers. Use small electronic components to limit background contribution. For ultimate background performance, bare-die JFETs and wire bonding are used.

CuFlon circuit boards

Canberra bare-die JFET configuration in C4

Screened polymer and cabling

ULB Gamma Spectrometer– 2017Germanium crystal integration

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 33

• Custom 2.7kg Canberra XtRa crystal (partial wrap-around Li contact)• 143% relative efficiency

Summary

Recent successes with the production of HPGe cryostats gives me confidence that we (The

Collaboration) can design and build a cryostat capable of achieving the background goals of

DAMIC-M.

Significant ultra-low-background capabilities and expertise exist at PNNL and are available to

DAMIC-M

There is a lot of work to be done before we will have a 90% cryostat design

For DAMIC-M to be successful the individual tasks will need to work together closely!

cryostat, shielding, CCD packaging, and electronics tasks

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 34

Questions?

June 27, 2018DAMIC-M Collaboration Meeting – June 11-13, 2018 – Paris 35

Heart of the Hill Vineyard – Red Mountain – Benton City, Washington