Instruments for x- and γ-ray astronomy...Cargèse, 5 April 2006 instrumentation 1 Instruments for...

Cargèse, 5 April 2006 instrumentation 1

Instruments for x- and γ-ray astronomy

Detecting x- and γ-rays

Detectors

Gas-filled detectorsScintillatorsSemiconductors

Telescope systemsGeometric Optics

Quantum Optics

Wave Optics


Instrument concepts in nuclear astrophysics

The instrumental categories in nuclear astrophysics reflects our currentperception of light itself.


Geometric Optics : Modulating Aperture Systems


Collimator "on" - "off" telescope : e.g. OSSE on CGRO


temporal modulation of a point source with a bigrid (Oda)-collimator

measured parameters :

Eγ : energy depositedt : arrival time

expected count rate on detector :N’(t) = + B

si : flux from the ith sourceε : detection efficiencyfi : transmission function for

source i at time tB : background count rate.

!i

si. .fi(t)


rotating modulator

gi(α) =

temporal modulation of a point source with a bigrid (Oda)-collimator

Transmission fi for a point source located at the position r,θ from theinstrument z-axis

fi = | 0.5 - (| gi-int(gi) |) |

with gi depending on the type of collimator movement and where int(gi) is theinteger part of gi

scanning modulator

gi(α) = r.cos( ) -

r.cos( - t)


Occultation Transform Imaging (BATSE)

with the planet earth as'rotation' modulation collimator(or scanning anti-collimator)


coded mask imaging


x,y : int. location on the detectorEγ : energy depositedt : arrival time

astronomy : encoding of a two dimensionalsource distribution (i,j) into a 2-D dataspace(k,l)

for sources at finite distance (nuclearmedicine, tomography of X-ray emittingplasmas) coded mask techniques can beused to extract depth information forvolumetric object reconstruction.


Why is it ...


Aristotle and the coded mask

“Why is it that when the sun passes through quadilaterals, as forinstance wickerwork, it does not produce a figure rectangular in shapebut circular ?”Aristotle,problemata physica - problem XV,6

“Why is it that in an eclipse of the sun, if one looks at it through asieve or through leaves, such as a planetree or other broad leavedtree, or if one joins of one hand over the fingers of the other, the raysare crescent-shaped where they reach the earth ? Is it for the samereason as that when light shines through a rectangular peep-hole, itappears circular in the form of a cone ? The reason is that there aretwo cones, one from the sun to the peephole and the other from thepeep-hole to the earth, and the vertices meet ..."Aristotle, problemata physica - problem XV,11


Field of view characteristics of a coded mask instrument

α2 β

2Ω2

ce

a

d

b

FOV (FWHM) = 2 arctg d2b

fully coded FOV = 2 arctg d-a2b

partially coded FOV = 2 arctg a+d2b

angular resolution = r ’

= r arctg cb

vignetting e,b, z from z axis


coded mask imaging : Encoding

The intensity measured by the PSD can be expressed as a two-dimensionalmatrix Di,j (the shadowgram) presenting the number of interactions registeredin the detector element i,j.

Dk,l = Σ Si,j . Ai+k ,j+l + Bk,l

Si,j : matrix of the source distribution,

Ai,j : aperture transmission function(1 for transparent mask elements, 0 for opaque elements)

Bi,j : background noise matrix(all contributions not modulated by the aperture)

i,j


coded mask imaging : Decoding

direct deconvolution :correlate the encoded matrix D with decoding array G (postprocessing array)

S’i,j =

Substituting the encoded matrix D results in

S’ = (S * A) * G + B * G

A*G is the point spread function (PSF). Optimal mask patterns produce deltafunction A*G ≡ δ

S’ = S + B * G

=> source is perfectly reconstructedd with the exception of a background term.

!k,l

Dk,l . Gi+k,j+l


“X-ray star camera”

Fresnel Zone Plate = Mask

⇒ Shadowgram = Hologram

Mertz & Young, 1961


“Illustrative sample of optical Fresnel transformation”

Mertz and Young’sdemo of the principleusing visible light :

upper left : sourceilluminated pinholessimulate the n stars

right : holograma Fresnel zone platecasts n distinctshadows

lower left : imagereconstructed bydiffraction from areduced copy ofhologram

Mertz & Young, 1961


I N T G E R A L

TITRE

INTEGRAL

Lebrun et al. (2003)


SWIFT

Detecting Area 5200 cm2Detector CdZnTeField of View 2 sr (half-coded)Detection Elements 256 modules of 128 elem.Detector Size 4 mm x 4 mm x 2 mmTelescope PSF 22 arcminEnergy Range 10-150 keVLaunch July 2004 !

BAT


Quantum Optics : e.g. Compton Telescopes


x1,y1 : interaction location in D1E1 : energy deposit in D1x2,y2 : interaction location in D2E2 : energy deposit in D2t, Δt : arrival time, TOF D1-D2

derived parameters :

x1,y1,x2,y2 => χ, ψE1,E2 => ϕ

cos ϕ = 1 - mec2/E2 + mec2/E1+E2

encoding of the two dimensional sourcedistribution into a 3-D dataspace (Χ, Ψ, ϕ)

-

-


the dataspace of classical Compton telescopes

Eventcircles from a single pointsourceat χ, ψ (35°, 0°)

events from same pointsource lie ona cone with apex at χ, ψ (35°, 0°)grayscale -> praobability density(for 1.8 MeV photons, max. at O(j,_) =23,7°)


Time of Flight coincidence (TOF) COMPTEL data

COMPTEL calibration data

channel width : 0.25 nsdistance D1-D2 : 1.5 m ≈> 5 ns)

COMPTEL flight data

channel width : 0.25 ns“upward BG” from spacecraft and the Earth


GRO COMPTEL


Advanced Compton Telescope (ACT) options

TIGRE MEGA Ge-ACT Liquid XEUC Riverside MPE, UNH UC Berkeley Columbia


ACT science requirements


Sensitivity

Variance V of a signal at the limiting flux fnV = (fn.X + N)1/2

N : number of equivalent total background counts, X : exposure.N = Non + α2Noff

= 2.b.Aon.h.ΔE.Tobs.soff.l for α = 1 X = Aon.Tobs.t.m.l.efep.sThe γ-ray source flux will be detected at n standard deviations is

fn = n . (fn.X + N)1/2 / X≈ n . N1/2 / X for fn.X fn = (n . N1/2/X) . [1 + ω+ 1/2ω2 + O(ω 4)] with ω := n/2 √ N


Background


Cosmic Ray interactions and γ-ray background


The background of our friendly skies ...

0.00

100

200

300

400

16 jan 12:00 18:00 17 jan 0:00 6:00 12:00 18:0018:00

Los Angeles to Toulouse

exp

osu

re r

ate

[m

icro

R/h

]

Toulouse time [h]

Los AngelesLAX

10±3 microR/h

ParisCDG

9±3 microR/h

ToulouseBlagnac

9±3 microR/h

New YorkJFK

11±3 microR/h

january 1998


Background - γ-Ray production within the atmosphere

0 10000 20000 30000 40000 50000 60000 70000

0

5000

10000

15000

20000

25000

30000

35000

40000

growth curve, HIREGS flight january 1998, McMurdo

count rate [counts / second]

alti

tude [m

]

BGO shieldLLD

BGO shieldULD

Ge det


Living with background - strategies

A : fightpassive shieldingactive anticoincidence shieldssupershieldsdiscrimination of BG-event signatures e.g.

- phoswhich- pulse shape discrimination (PSD)- time of flight measurements (TOF)

B : avoidchoice of orbit (e.g. high cutoff rigidity, avoiding radiation belts)minimize passive masschoice of low BG materials (e.g.70Ge)solid angle effects (earth -> high orbit, spacecraft -> mast)coincidence techniques (Compton telescopes, TPC’s)small detectors (focusing)resolution (spectral-, angular-, timing)


Background - Anticoincidence Shield

background reduction, shields against- prompt CR interactions- cosmic diffuse gamma-ray component- γ-rays generated in the atmosphere / spacecraft

defines a field of viewimproves spectral response function

- “Anti-Compton” rejection of source events not in full energy peak- Rejection of source events >mec2 in the escape peaks

background generator- converts CR p

reduces lifetimeimplicitly reduces effective area : with limited mass budgets in balloon and satellite

experiments, a shield is most likely the heaviest part of the instrument

( INTEGRAL SPI shield ACS : 856 kg ; BGO alone : 504 kg detector camera : 141 kg ; Ge alone : 19 kg )


Ge detector background : SPI

10-5

10-4

10-3

10-2

400 500 600 700 800 900

E [keV]

SPI 2003

BG

ra

te [count s

-1 c

m-3

keV

-1]

pvb 2

1.7

.2004

e+e-

(511)

24Na

(472)

69Zn

(439)

67Ga

(403)

69Ge

(574.1)

121Te

(573.1)

69Ge

(584)

74As

(596)

214Bi

(609)

72Ge(n,n')

(693)

117Te

(719.7)

126I ?

(753)

27*Al

(844)

54Mn

(835)

69Ge

(872)

69Ge

(883)

58Co

(811)

(818)


Ge detector background : SPI

10-5

10-4

10-3

10-2

400 500 600 700 800 900

E [keV]

SPI 2003

BG

ra

te [count s

-1 c

m-3

keV

-1]

pvb 2

1.7

.2004


Ge detector background : comparison

10-5

10-4

10-3

10-2

400 500 600 700 800 900

E [keV]

SPI 2003

Mars Odyssey

HEAO-3

HEXAGONE 1989

BG

ra

te [count s-

1 c

m-3

keV

-1]

pvb 2

1.7

.2004

HIREGS 1994

(ACS OFF)

TGRS

CLAIRE 2001

HIREGS 1994

SPI 2003

(ACS OFF)


Ge detector background : comparison

10-5

10-4

10-3

10-2

400 500 600 700 800 900

E [keV]

SPI 2003

Mars Odyssey

HEAO-3

HEXAGONE 1989

BG

rate

[count s-

1 c

m-3

keV

-1]

pvb 2

1.7

.2004

TGRS

CLAIRE 2001

HIREGS 1994

4 x

6 x


Anticoincidence shields - the limit of growth


ACT/GRI sensitivity requirement

f3σ < 5.10-7 s-1.cm-2

f3σ < 5.10-7 s-1.cm-2 !You must be kiddingthis means : ~ one photon per cm2 every month

with a BG produced by one CR particle per cm2 per secondproducing eg at 511 keV (SPI) ~ a BG event per cm2 every 3 minutes


Past, present, and future observations in gamma-ray lines


10-7

10-6

10-5

10-4

sensitivity

sensitivity

Past, present, and future observations in gamma-ray lines3 σ

nar

row

line

sens

itivity

at 8

47 k

eV

OSSEHEAO-3

SPI

required

sim

bo

l-X

XE

US

GR

I/M

AX

/AC

T

1960 1970 1980 1990

HE

AO

3

COMPTEL

HE

AO

1

OSSE

SMM

WATCH

SIGMA

BATSE

GINGA

GR

AN

AT

Bep

po

-SA

X

ASCA

Vela satellitesINTEGRAL

HE

TE

2000 2010

<SWIFT

AGILE

S

u

z

a

k

u

NEWTON, CHANDRA

EC

LA

IRS

10 MeV

1 MeV

100 keV

10 keV

1 keV


Gamma-ray source statistics

10.0

100

103

104

105n

um

be

r o

f so

urc

es

(19

98

/99

)

gamma-ray source catalogue



10-2

10-1

100

101

0.1 1 10 100 103 104

mass attenuation for NaI

attenuation [cm

2/g

m N

aI]

E [MeV]

pairCompton

photoel.

10.0

100

103

104

105n

um

be

r o

f so

urc

es

(19

98

/99

)




10-4

10-3

10-2

10-1

100

101

10-10

10-8

10-6

10-4

10-2

100

0.01 0.1 1 10 100

ba

ckg

rou

nd

[c

m-2 s

-1 M

eV

-1] (m

fp *

BG

)

Cra

b p

ho

ton

s [

cm

-2 s

-1 M

eV

-1]

energy [MeV]

OSSE

COMPTEL

EGRET

total Crab spectrum

background spectrum forGe detectorsspecific BG rate

times mean free path in Ge

0.00

50.0

100

150

200

0.1 1 10 100 103 104

nu

mb

er

of

so

urc

es

(19

98

)

energy [MeV]


Macom

b a

nd G

ehre

ls, 1999


Improving sensitivity

the detectable minimum flux will depend on the

Collecting area (for optical thick/thin detectors : detecting surface/volume)Detection efficiencyEffective exposure timeBackground

fn ≈ n.(N)1/2/X ≈ n.(bVTobs)1/2/AeffTobs

Building on present technology and measured BG spectra, the narrow line sensitivity ofan instrument with e.g. Aeff = 2000 cm2 (Ageo ≈ 1 m2) scales with respect to :

Aeff f3σ f3σ (X = AeffTobs = 2.109 cm2s)

OSSE 450 cm2 (662 keV) 8.10-5 4.10-5 [ph cm-2 s-1]COMPTEL 20-50 cm2 (2 MeV) 4.10-5 6.10-6 [ph cm-2 s-1]SPI 100 cm2 (847 keV) 2.10-5 4.10-6 [ph cm-2 s-1]

but how to get the ACT GRI requirements of ~ 5.10-7 [ph cm-2 s-1] ?

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