-decay: x y + azaz nn-2 a-4 4242 2 z-2 conservation of energy q for a parent a x nucleus at rest:
TRANSCRIPT
-decay: X Y + AZ N N-2
A-4 42 2Z-2
Conservation of Energy TTcmmmYYX 2)(
QFor a parent AX nucleus at rest:
m
p
m
pQ
Y
Y22
22
ppY
YYm
pmmTmmQ /1/12
2
AYm
m
QQT
411
Y
mmTQ /1
Note: for heavy nuclei QT
to within ~98% accuracy, anyway
We’ll see from a few examples that typically T 4-5 MeV
Is Pu unstable to -decay?23694
Pu U + 23694
23292
42 + Q
Q = (MPu – MU M)c2
= (236.046071u – 232.037168u – 4.002603u)931.5MeV/u
= 5.87 MeV > 0
Repeating an OLDIE but GOODIE from Lecture 13 on “Radiation”:
Some (especially the heaviest) nuclei are unstable with respect to the emission of heavy particles
•essentially the break up of a nucleus.
In one extreme: the emission of a single nucleon
but it includes the far more common alpha emission
and fission of the original nucleus into smaller, approximately equal sized nuclei.
Table 8.1 Energy Release (Q value) for various modes of decay of 232U
Emitted particle Energy Released (MeV)
n -7.26 MeV1H -6.122H -10.703H -10.243He -9.924He +5.51 MeV5He -2.596He -6.196Li -3.797Li -1.94
attractivenuclear potential
repulsiveCoulomb potential
The calculation of the kinetic energy of an alpha particle emitted by the nucleus 238U.
The model for this calculation is illustrated on the potentialenergy diagram at the right.
Let’s follow:
stepping through the details:
The mean binding energy per nucleon B/A for 238U
(from the Semi-empirical mass formula)is 7.5 MeV.
This potential energy curve combines a nuclear well
of radius 7.75 fm (from R = 1.25 x A1/3 fm)
and the Coulomb potential energy
of an alpha in the electric field of the daughter 234Th nucleus.
Thus to remove 4 average nucleons would require 30 MeV.
Compare to using thesemi-empirical mass formula
to calculate the energy required to remove
2 protons and 2 neutrons from the highest 238U
energy levels.
assumes they are the last two particles of each type added to the 234Th nucleus.
24.4 MeV
For the alpha particle m= 0.03035 u which gives 28.3 MeV binding energy!
The model for alpha emission proposes that the alpha particle is preformed inside the nucleus.
protons 2 1.00728 u
neutrons 2 1.00866 u
Mass of parts 4.03188 u Mass of alpha 4.00153 u
1 u = 1.66054 10-27 kg = 931.494 MeV/c2
Alphaparticle
NN
NN
An alpha particle with positive energy is created inside the nucleus
where it is trapped by the potential barrier.
According to quantum mechanics it has a finite probability of escape.
The binding energy released (28.3 MeV) appears in part as kinetic energy of the alpha.
Let’s see how well quantum mechanicsand our model of the potential
can calculate that probability (decay rate)
Nuclear potential
Coulomb potential
finite (but small) probability of being found outsidethe nucleus at any time
Tunneling
always some probability of a piece of the nucleus escaping
the nuclear potential
with a STATIC POTENTIAL this probability is CONSTANT!
Let’s examine this through a simple model: an forms inside the nucleus and then escapes through quantum mechanical barrier penetration.
The potential seen by the is spherically symmetric, so we can start by first separating the variables -
the functions Ylm are the same spherical harmonics
you saw for the wavefunctions for the hydrogen atom.
),()()( lmnl YrRr
02
)1()(/2/)(
2
2222
nlnl rR
mr
llrVEmdrrRd
Then the equation for the radial function Rnl(r) can be written as
For states without orbital angular momentum ( l = 0) this reduces to an equation like that for a 1-dimensional barrier.
eX where drErVm
)(2
2
In this case the inner limit of the integral is effectively the nuclear radius R, and the outer limit is taken as the point at which the ’s kinetic energy is equal in magnitude to its potential energy.
02
)1()(/2/)(
2
2222
nlnl rR
mr
llrVEmdrrRd
The transmitted part of the wave function X is of the form
The integral is carried out over the range of the potential barrier.
A solution can be found by approximating the shape of the potential as a succession of thin rectangular barriers.
xkxk DeCex 22)(II
In simple 1-dimensional case
E
I II III
V
)(2
22 EVm
k
where
222222II |)(| rkeDrx
In simple 1-dimensional case
E
I II III
V
probability of tunneling to here
x = r1 x = r2
R
E
Where
2
2
0
)2(2
4
1
r
eZT
r2
So let’s just write
r
eZrV
2
0
)2(2
4
1)(
Tr
rrV 2)(as
the point at which the ’s kinetic energy is equal to its potential energy.
hence drErVm
)(2
22
22 cos/ rr
drdr cossin2 2
1tan 2 r
r
then with the substitutions:
with E=T becomes drr
rmT1
22 2
2
drmT
drmT
222
22
sin22
2
)cossin2)((tan2
2
2221
0
2
2/1//cos
4
)2(2
22 rRrRrR
T
eZmT
and for R << r2 the term in the square brackets reduces to
2/22
rR
Performing the integral yields:
22 cos/ rr
2
2
0
)2(2
4
1
r
eZT
a
axxdxax
4
2sin
2sin2
2
0
2
2/2
24
)2(2
22 rR
T
eZmT
into which we can again substitute for r2 from
00
2
4
)2(82
2
)2(
ZmRe
T
meZ
2
2
0
)2(2
4
1
r
eZT
T
eZr
2
02
)2(2
4
1
and get
)2()2(
exp|)(| 2 ZRBT
ZAerX
When the result is substituted into the exponential the expression for the transmission becomes
The decay probability is = f X where f is the frequency with which the alpha particle hits the inside of the barrier. Thus
T
ZAZRBf
)2()2(lnln
f can be estimated from crude time between striking nuclear barrier
vv
rA )fm 6(2)(2 03/1
of 4-8 MeV “pre-formed” alpha
second/1022 timesf
Easily giving estimates for = 106/sec – 10-21/sec
Some Alpha Decay Energies and Half-lives
Isotope T(MeV) 1/2 (sec-1)
232Th 4.01 1.41010 y 1.61018
238U 4.19 4.5109 y 4.91018
230Th 4.69 8.0104 y 2.81013
238Pu 5.50 88 years 2.51010
230U 5.89 20.8 days 3.9107
220Rn 6.29 56 seconds 1.2102
222Ac 7.01 5 seconds 0.14216Rn 8.05 45.0 sec 1.510
212Po 8.78 0.30 sec 2.310
216Rn 8.78 0.10 sec 6.910
T
ZAZRBf
)2()2(lnln
should be compared with the emperical Geiger-Nuttall law
DTC lnln
this quantum mechanically-motivated relation
The dependence of alpha-decay half-life on the kinetic energy of the alpha particle.Values are marked for some isotopes of thorium.
232ThQ=4.08 MeV=1.4×1010 yr
218ThQ=9.85 MeV=1.0×10-7 sec
For each series of isotopes theexperimental data agree (1911)
The potential seen by an electron in the hydrogen atom
is spherically symmetric (depends only on r, not its direction)!
r
erU
2
04
1)(
),,(),,()(sin
1sinsin
sin
1
2 2
22
2
2
rErrUr
rrr
To solve we apply a separation of variables: )()()(),,( FPrRr
Recognizing that we write Schrödinger’s equation in spherical polar coordinates
r
erU
2
04
1)(
),,(),,()(sin
1sinsin
sin
1
2 2
22
2
2
rErrUr
rrr
)()()(),,( FPrRr with
)()()()()()()(
sin
1)()(sin)()(sin)()(
sin
1
2 2
22
2
2
FPrERFPrRrU
FPrR
PFrR
r
Rr
rFP
r
ErUF
F
P
Pr
Rr
rrRr
)(sin
1
)(
1sin
)(
1
)(
sin
sin
1
2 2
22
2
2
ErUF
rF
P
Prr
Rr
rrRr 222
2
2222
2
2)(
2
sin
1
)(
1sin
)(sin
1
)(
1
ErUF
rF
P
Prr
Rr
rrRr 222
2
2222
2
2)(
2
sin
1
)(
1sin
)(sin
1
)(
1
ErUF
FrYrX 22
22 2)(
2
)(
/),(),(
),()(
/ 22
rZF
F
= K (some constant)
ErUKr
P
Prr
Rr
rrRr 222222
2
2)(
2
sin
1sin
)(sin
1
)(
1
ErUQrr
Rr
rrRr 2222
2
2)(
2)(
1
)(
1
)())((2
)(
12
22
QrUEr
r
Rr
rrR
= K2 (also some constant)
Then the problem becomes finding solutions tothe separate “stand alone” equations
each of which uniquely constraints the wavefunction:
A solution to the radial equation can exist only when a constant arising in its
solution is restricted to integer values (giving the principal quantum number)
Similarly, a constant arises in the colatitude
equation giving an orbital quantum number
Finally, constraints on the azimuthal equation
give a magnetic quantum number
Let’s examine this through a simple model: an forms inside the nucleus and then escapes through quantum mechanical barrier penetration.
The potential seen by the is spherically symmetric, so we can start by first separating the variables -
the functions Ylm are the same spherical harmonics
you saw for the wavefunctions for the hydrogen atom.
),()()( lmnl YrRr
02
)1()(/2/)(
2
2222
nlnl rR
mr
llrVEmdrrRd
Then the equation for the radial function Rnl(r) can be written as
x
x
y
y
z
z
x
y
z
y
z
x
y
z
x
y
z
x
x'
y'
x'
y'
z'
PARITY TRANSFORMATIONS ALL are equivalent to a reflection (axis inversion) plus a rotation
The PARITY OPERATOR on 3-dim space vectors
every point is carried through the originto the diametrically opposite location
Wave functions MAY or MAY NOT have a well-defined parity(even or odd functions…or NEITHER)
xcos xx cos)cos(P P = +1
xsin xx sin)sin(P P = 1
but the more general
xx sincos xx sincosP
However for any spherically symmetric potential, the Hamiltonian:
H(-r) = H(r) H(r)→ →
[ P, H ] = 0
So they bound states of such a system have DEFINITE PARITY!
That means, for example, all the wave functions of the hydrogen atom!
azrea
z /23
100
1
aZrea
Zr
a
Z 2/23
200 21
2
1
iaZr erea
Z
sin
8
1 2/25
121
cos2
1 2/25
210aZrre
a
Z
iaZr erea
Z sin
8
1 2/25
211
aZrea
rZ
a
Zr
a
Z 3/2
2223
300 21827381
1
108
6
4
3
2
1
-dE
/dx
[ MeV
·g-1cm
2 ]
Muon momentum [ GeV/c ]0.01 0.1 1.0 10 100 1000
1 – 1.5 MeVg/cm2
Minimum Ionizing:
-dE/dx = (4Noz2e4/mev2)(Z/A)[ln{2mev2/I(1-2)}-2] I = mean excitation (ionization) potential of atoms in target ~ Z10 GeV
The scinitillator responds to the dE/dx of each
MIP track passing through
A typical gamma detectorhas a light-sensitive
photomultiplier attachedto a small NaI crystal.
If an incoming particle initiates a shower,each track segment (averaging an interaction length)will leave behind an ionization trail with about the same energy deposition.
The total signal strength Number of track segments
Basically avg
MIPtracksmeasuredENE
Measuring energy in a calorimeter is a counting experiment governed by the statistical fluctuations expected in counting random events.
Since E Ntracks and N = N
we should expect E E
and the relative errorE E 1
E E E =
E = AE
a constant that characterizes the resolution
of a calorimeter
(r)=(r)ml () the angular part of the solutions
are the SPHERICAL HARMONICS
ml () = Pm
l (cos)eim
Pml (cos) = (1)msinm [( )m Pl (cos)]
(2l + 1)( lm)! 4( l + m)!
d d (cos)
d d (cos)Pl (cos) = [( )l (-sin2)l ] 1
2l l!
The Spherical Harmonics Y ,ℓ m(,)
ℓ = 0
ℓ = 1
ℓ = 2
ℓ = 34
100 Y
ieY sin
8
311
cos4
310
Y
ieY 2
2
sin2
15
4
122
ieY cossin
8
1521
2
12cos2
3
4
1520
Y
ieY 3
3
sin4
35
4
133
ieY 2
cos2
sin2
105
4
132
ieY 12cos5sin
4
21
4
131
cos
2
33cos2
5
4
730Y
then note r r means
x
y
z
and: Pml (cos) Pm
l (cos()) = Pml (-cos)
(eim=(1)m
so: eimeimeim
but d/d(cos) d/d(cos)
(-sin2)l = (1cos2l
ml () = Pm
l (cos)eim
Pml (cos) = (1)m(1-cos2m [( )m Pl (cos)]
(2l + 1)( lm)! 4( l + m)!
d d (cos)
d d (cos)Pl (cos) = [( )l (-sin2)l ] 1
2l l!
So under the parity transformation:
P:ml () =m
l (-)=(-1)l(-1)m(-1)m m
l ()
= (-1)l(-1)2m ml () )=(-1)l m
l ()
An atomic state’s parity is determined by its angular momentum
l=0 (s-state) constant parity = +1l=1 (p-state) cos parity = 1l=2 (d-state) (3cos2-1) parity = +1
Spherical harmonics have (-1)l parity.
In its rest frame, the initial momentum of the parent nuclei is just its
spin: Iinitial = sX
and: Ifinal = sX' + s + ℓ
1p1/2
1p3/2
1s1/2
4He
S = 0 So |sX' – sX| < ℓsX' + sX
the parity of the emitted particle is (1)ℓ
mY ~
Since the emitted is described by a wavefunction:
Which defines a selection rule: restricting us to conservation of angular momentum and parity.
If P X' = P X then ℓ = even
If P X' = P X then ℓ = odd
If the 2p, 2n not plucked from the outermost shells(though highest probability is that they are)
then they will leave gaps (unfilled subshells) anywhere:Excited nuclei left behind!
EXAMPLE:
If SX = 0
|sX' – sX| < ℓsX' + sX
ℓ=sX' (conservation of angular momentum)
0 3 nuclear transition would mean ℓ=3 so PX' = PX
i.e. 0+3 is possible, but
0+3 is NOT possible
0 2do not change the parity of the nucleus
0 4so PX' = PX
So 0+2
0+4would both be impossible